October 15, 2019 • 37 mins
What is color and how do we perceive it? How many colors are there? UCI particle physicist Professor Daniel Whiteson explains the science of colors and answers some listener questions as well.
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Speaker 1 (00:08):
When I was a kid, I was fascinated by color,
and in particular, there was one question which had me
up late at night thinking about it, which was this,
can you think up a new color? Now? If you've
seen a rainbow, then you know that the whole spectrum
of visible light is reflected there. You have all the reds,
all the oranges, all the greens, all the yellows, all
(00:30):
the way up to the blues and the violets, and
you can see they're all of the colors that you
can perceive. And of course it makes you wonder about color,
How does color work? What is it really? And it
also connects to some deep questions about philosophy, not just physics.
For example, many people have wondered the color that I'm
seeing red, how do I know that other people are
(00:50):
seeing the same color? Right? Maybe the thing that I
see is red somebody else sees as blue. Fascinating question
in philosophy. But there's another question there, which is can
you think up a new color? If these colors that
I'm seeing are just perceptions in my mind? Is my
brain capable of coming up with a new color? Can
(01:11):
I generate in my own head a new experience of color.
I spent many nights thinking about whether it was possible
to concentrate hard enough to come up with a new
color Hie. I'm Daniel. I'm a particle physicist and a
(01:43):
part time podcast host and the co author of the
book We Have No Idea, A Guide to the Unknown Universe,
which takes you on a tour about all the things
we don't understand about the universe. And you're listening to
the podcast Daniel and Jorge Explain the Universe production of
My Heart Radio. My co host Jorge Ham and co
author in that book, can't be here today, so I'm
(02:05):
talking to you on my own about all the amazing
things in the universe. Our podcast tries to find incredible,
mind blowing, really hard to think about things and explain
them to you in a way that you actually understand
and maybe even entertains you along the way. Today on
the program, we're gonna be walking a fine line between
physics and philosophy, because there's a deep connection between these fields.
(02:28):
Sometimes in physics we discover something that reveals the truth
of the universe, and that truth can make us feel
differently about our relationship with life and the universe and
how everything works. This, of course is true when we're
talking about the beginning of the universe and how it
all came to be in its potential future end, but
also about how we perceive the universe, the very everyday thing,
(02:50):
and one of the most tangible ways we have to
perceive the universe, of course, is with light, and specifically
with color. Color is so physical, it's so tangible, it's
so such an intense experience. But what is it? What
does physics have to say about color? And so that's
the topic we're gonna be tackling on today's podcast. What
(03:14):
is the physics of color? And there's lots of different
aspects to this question. How many colors are there? Why
do we see things in different colors? Why some objects
different colors than other objects? Has it all work? And
there's a great history here of physicists diving into color.
Even Isaac Newton did some of his original best work
with lenses and optics and prisms, and he studied spreading
(03:37):
of white light into the rainbow. And in the early
part of this century, color was a big clue that
helped us understand quantum mechanics. People saw all sorts of
weird patterns that they didn't understand, and it took some
clever brains and some interesting experiments to untangle it. Now,
everybody has some understanding of color. Everybody has some experience
(03:57):
of color. Well, some people out there might be color blind.
But does everybody understand color? Do people know how color works?
Why we see things different color, why some things reflect
blue and other things reflect green? Do people really understand
what color is? So to get a sense of the
general level of knowledge of color, I walked around this
time in Aspen, Colorado, and I asked people what they
(04:19):
knew about color and light and why different things were
different colors. Listen to what they have to say, but
first think to yourself. Do you understand color? Do you
understand light? Do you understand why things are different colors?
Can you imagine a new color in your mind? Think
about those things as you listen to these answers. I
couldn't tell you that one something about the light, but
(04:41):
I don't know reflecting light. I don't know the spectrum
from the sun. There's infrared colors. We can't see the colors.
We can see the spectrum of the colors and the
light causing, uh, what you see for colors? I don't
know physical light is a certain wavelength. Your I C
(05:03):
is visible light and has more to do it's the
light bouncing after the objects. I also don't all. I'm sorry,
it's something to do with the light with our eyes
the color of white. So you're probably hear in those
answers that there's definitely some understanding of light and wavelengths
and color, and that there's definitely some physics to it. Right.
(05:25):
People understand that behind color is a lot of physics,
and that's great because we're gonna dig into all of
that physics today. But there's not a lot of understanding
for why different things are different colors. Why is this
bench blue, why is the grass green? All of these things?
How does that work? On a sort of microscopic level.
One of my favorite things about physics is that we
(05:45):
can take the macroscopic universe, the one that we experience,
and take it apart and explain it in terms of
microscopic stuff. We understand the difference, for example, between frozen
water and liquid water in terms of the motion of
the little articles inside, and everything we are experiencing around
us is in the end, just an emergent phenomenon of
(06:06):
these microscopic events, And so we'd like to understand basically
everything around us in terms of the microscopic principles. Right,
what is really happening on the tiniest level that makes
something green or makes something else red. And by the
end of today's podcast, I hope you'll have a solid
understanding of why things are different colors. So let's dig
into it first. What is light and what is color? Well,
(06:29):
let's begin with light. Light, of course, is just electromagnetic radiation.
We talked about this on the podcast fairly often. You
go all the way down from radio waves up to
gamma rays and X rays. All of these things are
just electromagnetic fields that are wiggling. That's why they can
get to you across the vast distances of space. It's
not like sound, where you have the air that's shaking.
(06:51):
Light is the waving of electromagnetic fields. And electromagnetic fields
are a property of space itself, like all the quantum
fields that we talked about on another podcast, Every element
of space has the possibility to have light in it,
or electrons in it, or any of the other quantum fields.
So when a photon passes through space, what that really
(07:11):
means is the electromagnetic fields in that space are oscillating,
and so all kinds of light are just electromagnetic radiation.
Right in the middle of the spectrum is visible light
at about a few hundred nanometers and it's no different
from the light at higher energies and lower energies, except
of course, for that energy. So the properties that a
(07:31):
photon has that you need to understand are just its energy. Now,
its energy is very closely connected to its frequency. The
more energy the photon has, the faster it wiggles, and
the faster it wiggles, the shorter its wavelength. All these
photons have the same speed. They all travel the speed
of light, but they have different amounts of energy per photon.
(07:53):
And every photon you can translate its energy directly into
its frequency, and its frequency directly into its wavelength. It's
really just one piece of information expressed in different ways.
I want to talk a little bit more about that,
but first let's take a quick break. So photons, of course,
(08:21):
are quantum mechanical particles. We've talked on this podcast many
times about how they can be seen as particles, they
can be seen as waves, and it's true, and you
should think about them as quantum mechanical, but only in
the sense that you can only have a certain integer
number of photons. Like you turn on your laser. You
can have one photon, or two photons, or three photons
or seventy four photons. You can't have one and a
(08:44):
half photons. You can't have two point seven two photons.
That's where the quantum mechanics comes in. It's it's a
discrete number of photons. But this other property of photons,
the energy of a photon equivalent again to its frequency
and therefore wavelength that can have any value. A given
photon can have any amount of energy, from very very low,
(09:07):
making it like a radio wave, to very very high,
making an X ray or a gamma ray. We'll talk
later about how photons are generated, and there are some
objects that can only generate photons of certain energy, but
in principle of photon can have any energy. What that
means to the context of color is that every individual
photon can have any energy level, which means you could
(09:27):
have any frequency, which means it could have any wavelength,
And of course the wavelength of the photon is connected
to the color. We perceive photons of different wavelength as
having different colors than a four D nimeters. For example,
we perceive things as very very red up but seven
d nanometers we perceive things as very very blue or
(09:47):
very very violent. So there's a close connection between the
wavelength of the photon and the color that we perceive.
But don't be confused. The color is not a property
of a photon. Sure people say a red photon, but
what they mean is that the photon has a certain wavelength.
We perceive it as red, but the redness is inside us.
(10:10):
There's nothing red about the photon. The photon just has
a certain wavelength. So when we're talking about the physics
of color, let's separate what property the photon has and
our perception, our experience of it. Alright, so back to
the photon. You can have any infinite number of different
wavelengths for a photon. What that means is that potentially
(10:33):
it is an infinite number of colors. If every wavelength
corresponds to a color, then there's an infinite number of
colors out there. Can we perceive an infinite number of colors.
Let's talk for a moment about what about how we
perceive color. Imagine this spectrum of different wavelengths from four
d nimeters up to seven hundred nimeters. Yes, there's an
(10:54):
infinite number of different wavelengths you could stick into that spectrum. Right,
it's the real numbers and an infinite number. Just the
same way, there's an infinite number of numbers between one
and two. Right, there's one, one point one, one point one, one,
one point one, seven, etcetera. I could go on literally
forever and name numbers between one and two. In the
(11:15):
same way, there's an infinite number of wavelengths photons can have,
but we are limited in how we can perceive them.
We can't necessarily tell the difference between two slightly different
wavelength photons. We might perceive them the same way. That's
just a matter of resolution. It's like in your camera
has a certain number of pixels, and so something falls
(11:36):
in one pixel, you can't tell where in the pixel
it landed. Did it land in the center, did it
land towards the edge. You can't tell because your camera
has a certain spatial resolution, a certain number of pixels,
and of course the more pixels it has, the better
it at it is at figuring out exactly where those
photons landed. In the same way, your eye is not
(11:56):
capable of distinguishing between every time need a little difference
in wavelengths. Two photons that have almost the same wavelength
will register exactly the same way in your eye. Now,
in your eyeball, there are actually three different kinds of
cells in the back of the eyeball that's see color.
What they do is they respond differently to photons at
different wavelengths. One of them peaks very very low, it's
(12:19):
mostly responsive around four hundred and fifty nanimeters. The second
kind peaks sort of in the middle of the spectrum,
like five hundred fifty nanimeters, and the third kind peaks
a little bit higher, just under six hundred nanimeters. So
I have three kind of cells. Each one is sensitive
to different wavelength photons. So the way that it works
is that the photon hits your eyeball and then some
(12:41):
of these light up. If the photon has a wavelength
which corresponds to the peak of the sensitivity for one
of those cells, it'll light up really strongly, like the
one at four fifty. If you send a photon and
at four hundred fifty nimes right at that one, it's
gonna light up, And the other ones are not going
to light up very strongly, whereas if you send a
photon around six hundred nimes, then the third kind is
(13:02):
going to light up really strongly and the other two
are going to be dimmer. And then your brain takes
that information. It says, the low wavelength one lit up
and the other two didn't, so therefore the light we're
seeing must be low wavelength, or if it gets messages,
it say that only the high wavelength sensor lit up,
and then it knows that the light you're seeing must
(13:22):
be high wavelength. It's not that your your eye specifically
measures the wavelength of any individual photon. What it does
is it asks how much does it light up each
of these three sensors, and then it has to reverse
engineer and estimate what was the wavelength of the light
that hit it. It's sort of like triangulation. Your cell
(13:44):
phone knows where it is because it can talk to
like three different cell phone towers, and it can ask
those towers how far away from you am I? And
if one of the tower says, oh, you're real close,
and the other two say no, you're pretty far, then
your phone knows it's pretty close to one of those towers,
and it can tell exactly where it is because it
has the messages from all three. That's called triangulation. Well,
(14:06):
your eye is doing the same thing with the sensors
in the eyeball. It gets three pieces of information about
the light that's coming in, and each of those gives
it different information, right, information about how close are you
to the wavelength that this sensor is good at seeing,
and then it can use that information to decide what
wavelength of the light actually hits you. So the final
(14:27):
perception is sort of mixed from these three different measurements
we make. And this is why you can build up
any sort of color that humans can experience out of
just three sort of basis colors. You often hear about
the primary colors or red, green, and blue, and any
color the humans perceive can be built up with some
combination of red, green, and blue. And this blew my
(14:50):
mind the first time I thought about it. I thought, Wow,
there's like colors live in some sort of mental, abstract
mathematical space, and red, green, and blue, or like the eye,
the invectors of it, and any color you can imagine
is just a linear combination of those three colors. That
was incredible to me, but it's not actually true that
just encompassed the human experience of color. Remember, there's an
(15:13):
infinite number of colors in the spectrum because it's an
infinite number of wavelengths. What r GB does is it
plays with human responses. We have three ways to measure colors,
and so it triggers those three sensors in different ways
to give you the experience of different colors. The same
way in the case of the cell phone towers, if
you could pack those cell phone towers with different distances,
(15:34):
you could stimulate being any place between those towers in
that same way. All right, So to recap photons have
any arbitrary wavelength, which is which is controlled by the
energy that they carry. And if we imagine the relationship
between wavelength and color, color is part of the human perception.
(15:55):
Color is what we experience. There's nothing red about the photon.
Per se. It has a certain wavelength which your brain
measures your eyeballs like a device for measuring the wavelength
of those colors by using those three different sensors to
triangulate it, and then it gives you the experience of
that color. But because there's nothing in particularly red about
(16:16):
the photon. Where does redness come from? And this is
where physics crosses into the realm of philosophy, or physics
inspires fascinating questions in philosophy. And one of the really
interesting wrinkles here is that not everybody out there has
three color sensors in their eye. There are some folks
(16:38):
out there that have a mutation. They have four kinds
of sensors in their eye. They are called tetra chromats,
and they have an extra way to sense color in
their eye. When I first learned about this, I thought, oh,
does that mean that they have like another color in
their mind? Is this fourth kind of cell that can
detect another element of the spectrum give them a new
(17:01):
kind of experience that I can't have. Is there some
color out there that they can experience that I will
never know? That's not the case. Actually, it's just a
fourth way of sensing the wavelength of the light that
you're seeing. So it gives them better ability to nail
down the wavelength of the light. They don't necessarily see
any new colors. It's like adding a fourth tower to
(17:23):
your triangulation. It gives it helps you separate in cases
where it's hard to tell. It gives you extra information
to tell where that cell phone is, it doesn't necessarily
give you a totally new experience of distance. So tetrachromats
are interesting and fascinating, but they don't necessarily see color
differently than we do. They're just better at it. It's
(17:44):
like if you're measuring the length of something and your
ruler has more little marking, so you can make a
more precise measurement of the length of whatever it is
you're looking at. All right, but back to the sense
of philosophy, perception has to be something in the mind,
because again, there's nothing blue or purple or orange about
the photon. That's something that your brain is doing. And
(18:04):
that's why, of course people wonder, is the read that
I'm experiencing different from the red that you're experiencing. Maybe
the red that I'm experiencing is you're blue. That seems
unlikely because we all sort of like the same kind
of art and the same kind of combinations of colors.
But we don't really know, because we can never really
experience what's in somebody else's mind. And this is a
(18:25):
famous question in philosophy. Can you describe redness? Can you
communicate somehow. Is there any possible way to capture the
experience of redness to convey that to somebody else without
them experiencing your redness? Can you describe redness in other terms?
Or is it unique? Is it? Is it its own
(18:46):
sort of basis concept in the idea structure of your mind.
And there's this famous thought experiment. Say you take a
genius scientist and you put her in a room, and
and the scientists only ever see black and white, and
she can learn all about the world, and she can
learn all about science, but she only ever sees black
and white. There's only black and white. Thinks in the room,
(19:07):
and the TV she's using only has black and white,
so she never sees any color. And this person is
super duper smart. Is there any way that she can
understand color so that when she opens the door and
you let her out of this terrible mind experiment you
can never actually do on people. So when she emerges
into the world and sees color and experiences it for
(19:28):
the first time, she will have already understood. Is there
any way to give her that understanding without the experience.
If so, then it means that color is something that
you can translate into other ideas and convey from mind
to mind. If not, then it means it's something purely internal,
something that cannot be described in any other way, meaning
(19:51):
that we can never know if my red is the
same as your red. So it's a famous unanswered question
and philosophy. But it stimulates in me another question, which
is how many colors are there in our mind? I mean,
if they are just in our mind, If the red
that I'm seeing as I look at this T shirt
right now is something an experience that my brain is
(20:12):
generating for me, then can it also generate other colors?
Obviously you can. You can generate blue, can generate orange,
You can generate purple. Right. It takes some external stimulation
to make that happen. But the generation and the experience
itself is in my mind. It's after the information from
the wavelength has been transformed into some sort of pulse
(20:33):
in my brain, and that's when the experience of purple happens.
So then the question is could I generate a novel
one could I think of? Could I imagine new color
that nobody has ever imagined, or at least that I
have never imagined? You know, say I had never seen
anything green before in my life. I had only ever
seen red and blue. Could I think up green? Could
(20:56):
I envision green in my mind without having ever seen it?
Or even if I've seen red, green and blue, can
I come up with a new color? So I honestly
spent many afternoons as a kid trying to come up
with a new color, and it always ended up something
weird and orange. But I never succeeded, and so to
this day, I still do not know the answer to
that question. So that tells us a little bit about
(21:20):
the physics of color. What is color? How is it
connected to the wavelength of light and electromagnetic radiation, and
how we perceive it and what that means. How you
translate from the photons that are out there in the
universe to our perception of color, which is fascinating, but
it doesn't tell us about what's happening microscopically in stuff.
Why is that shirt blue? In this shirt red? Why
(21:42):
is it generating photons at different colors? So I see
those things, so I experience those We'll dig into all that,
but first let's take a little break. Okay, we're talking
(22:04):
about the physics of color and the experience of color.
And this is something which goes back to the early
nine hundreds when there was a really interesting scientific puzzle
that people were trying to understand, which is that some
gases have color. You've probably experienced this if you've ever
played with like a Bunsen burner and put some weird
stuff in it and you see it, Oh, it glows green.
(22:25):
Or if you put this metal in it, you get
something purple. If you put this metal in it, you
get a red flame. And so fire has different colors.
And remember the fire is just essentially ionized gas. You're
heating something up and it's glowing and emitting photons, and
that's what you're seeing. But back on the day before
we had a really detailed understanding of the quantum mechanics
of it, people were wondering why do different gases have
(22:46):
different colors? And more specifically, there were two things that
people noticed. First of all, they noticed the gases absorbed colors.
So if you're shown, for example, a white light through
a bunch of gas, you measure the wavelength of the
light that came through, you'd notice that the gas absorbed
certain wavelengths, but only certain wavelengths, and it depended on
(23:08):
the gas. Nitrogen would absorb different things than hydrogen would
absorb different things than oxygen. So each gas seemed to
have its own pattern. These little slices of the spectrum
that we get taken out of the white light when
they pass through the gas. So you pass white light
through a gas and it removes a certain little slices
of that spectrum, and it's like a fingerprint. You can
(23:29):
tell what gas is there based on which slices of
the spectrum it takes out. But nobody understood why does
this gas take out that those colors and why does
that gas take out the other colors? And the second
thing is the inverse of that. You took those same
gases and you heated them up, and they would glow,
but they wouldn't glow in every color. They don't glow
(23:51):
white necessarily. They glow in certain colors. And the colors
they glow in match exactly the colors that they would
take out of the spec drum when you pass white
light through them. So, for any particular gas, if you
passed white light through it, it would slice out little
parts of the spectrum. But then if you took that
same gas and you heated it up, it would emit
(24:12):
light and exactly those little wavelengths that it had sliced out.
So something interesting was going on. And before people understood
the microscopic physics of it, there was a lot of
study and just a lot of sort of thought about it.
People measured the wavelengths, of course, very carefully and did
detailed experiments to try to understand it, because data, of
course is the source of insight and much of the science,
(24:33):
and especially in physics, and a lot of mathematicians looked
at those spectrum and they noticed patterns. They noticed that
there was spacing between the wavelengths that the that the
gases would absorb, and they saw these patterns that the
spacing would grow larger and larger and larger, and they're
able to fit mathematical equations to those spacings. Now, they
didn't understand where those equations came from, but they noticed
(24:55):
that they were there. So Ridberg, for example, came up
with this formula, and he had no understanding for what
causes formula could couldn't explain the formula at all, but
it worked perfectly. And that's a great clue because it
tells you what's the mathematical structure and the end physics
is always trying to describe the universe in terms of mathematics.
(25:15):
The stated goal of physics, of course, is to write
down an equation that describes everything in the universe and
then look at that equation and understand from the mathematical
structure of that equation, what do we learn about the
nature of the universe. So math is our language. So
as soon as we can turn a big pile of
data into sort of a compressed mathematical equation, then we
(25:36):
can ask questions about the structure that equation and wonder
why is it this way, what is it? Why is
it that way? And it was Neil's bore that figured
it out when he built his atomic theory, the one
that has little electrons orbiting the center. And of course
that's passe because we don't think these days about electrons
orbiting because they're not classical objects that have paths, and
(25:56):
we'll dig into that in a future episode about quantum mechanics.
In his model, electrons were orbiting the nucleus of the atom,
and they could only have certain energy levels. And what
happened when an electron jumped down an energy level It
had to give up some of that energy, and it
gave up that energy in terms of a photon. So
if an atom has certain restricted energy levels, then the
(26:19):
electron can jump down only certain distances, and those distances
correspond to the energy of the photons that can be
emitted by that atom, and therefore correspond to the wavelength
of the light that you see. So if you take
any particular atom, it has certain energy levels, and if
you heat that up, then the electrons jump up energy levels.
(26:39):
They're absorbing that energy, and then sometimes they jump down,
and when they jump down they give off those photons.
So that explained why certain gases emitted only in certain spectrum,
and every gas has its own particular set of wavelengths
that it can emit in those wavelengths again controlled exactly
by the difference in the energy levels of the electrons
(27:01):
going around the center. And it also, awesomely also explained
the absorption because if you take white light and you
shine it at that gas, it can't absorb any arbitrary photon.
It can only absorb photons that will take the electron
up one energy level, or two energy levels, or three
energy levels. And it's this mathematics that explain that spectrum
(27:25):
that the electrons have to move up or down one
or two or three steps, no half steps, no quarter steps,
no one point to seven steps that determine which photons
the atoms can absorb and can emit. So that helps
us understand sort of the physical basis of why different
things give off different colors, why different things look different colors.
(27:46):
So let's put it all together. You have light from
the sun. Now, life from the sun is in lots
of different frequencies, is a broad spectrum. It peaks in
the yellow or sometimes people say it's a little bit green,
but mostly you have life from the sun all all
over the visible spectrum. And that's not a coincidence that
the sun happens that you give off photons in the
same spectrum that we can see things. Right, our eyes
(28:08):
evolved in the presence of this sun in order to
be able to see photons which were around us. We
can think of it as evenly spread across all of
the wavelength. Now, what happens when that light hits your
red T shirt. Well, when light hits your red T shirt,
it gets reflected off the T shirt, but not entirely.
(28:29):
Some of the colors of that white light get absorbed
by your red T shirt. Why does your red T
shirt absorbed only some colors Because the atoms in your
red T shirt have electrons which can jump up one
energy level and accept photons at just the right wavelength. So,
just like the gas where if you pass white light
through it, it will delete certain wavelengths, your T shirt
(28:52):
will delete a bunch of wavelengths from white light. And
your T shirt is red not because it's absorbed photon
on which are in the red part of the spectrum,
but because it's reflected them. This is a common misperception.
People think white light comes from the sun and your
T shirt is red because it's absorbed the red parts
and reflected everything else. Remember that you are seeing photons
(29:14):
only when they hit your eyeball, and so I see
your shirt is red because your shirt has reflected those
red photons. To me, right, light is something I'm experiencing
based on the photons that are being reflected or emitted
from an object, not some like inherent property that it has.
Something absorbs red photons, It doesn't turn that object red.
(29:36):
To see something as red, you have to see red
photons leaving it, which means they have to reflect from
that object. So something that's blue, for example, absorbs red photons.
Something that's red absorbs blue photons to a little bit
backwards right. Or more specifically, something that looks blue absorbs
photons of every wavelength except for blue. Something that looks green, mean,
(30:00):
absorbs photons of every wavelength except for green. And this
is a model of color we call subtractive color because
you start from the white light, which is every kind
of wavelength, and you remove stuff. When something hits your
blue t shirt, a bunch of photons get absorbed, right,
they get removed, So we call that subtractive color. There's
another way to think of color, and that's additive color.
(30:23):
Instead of its starting from full white light and talking
about the color you perceive. If you start from nothing,
you start from blackness, for example a computer monitor, as
opposed to a piece of paper. Start from a computer monitor,
then you can add light to make various mixtures. But
but it's a little bit complicated. The two different ways
of thinking about light are fundamentally equivalent in the end.
(30:45):
But if you design something, for example, on your computer monitor,
and then you print it out on a white piece
of paper, it might look a little bit different from
you expected. So those are you out there who are
artists know all the details about the difference between subtractive
color models and additive color models. All right, So we've
been talking about color and photons, and now I think
we have a pretty good understanding of the physics of it. Remember,
(31:07):
photons have certain wavelength which corresponds to their energy, and
they're just flying around the universe having a certain energy
per photon. The experience of color is something that happens
inside our brain, is the interpretation of signals along the
optic nerve that comes from the eyeball. The eyeball has
done its best to measure the wavelength of the light
that's hitting it. But the experience of color is something internal,
(31:29):
something in the mind, something that philosophers can probe and
physicists can wonder about. But it also makes us wonder
what it's like to experience the world, and whether we
could see the world differently if we had different kinds
of eyeballs. So we've got a great question from a
listener which I want to actually answer right now. Here's
the question. Hi, Daniel Hire, This word that looks pretty
(31:51):
good and sharp in the visible spectrum, of light. But
what would it look like if you could only see
lowware or eh our frequency is off flight? Would a
low frequency world be all transparent? Thank you, what a
great question. I love imagining alternative universes where we had
different kinds of eyeballs or different kinds of experiences. So
(32:14):
it's an interesting question and actually one that you could
answer yourself because we have technology for this. For example,
night vision goggles do this sort of frequency shift, and
they'll let you see light that's out there that your
eyeballs cannot measure. They'll let you see at night because
there are actually photons flying around just that your eyes
cannot see them, in the same way that like infrared cameras.
(32:38):
Infrared cameras see photons that are have too long a wavelength,
wavelength that your eyes cannot see, but that are out there.
And so in the infrared, the world certainly does look different.
Have you seen the Predator movies, for example, where you've
seen any sort of military action movie, you know that
infrared you can see people's heat, you can tell what's
(32:58):
hot and what's not because things glow in the infrared
when they're hot, and so you can definitely have a
different experience of the world if you could see a
different wavelengths, and yes, different things would be transparent and
different things would be opaque because the opacity of something
and its transparency is a function of its wavelength. Right,
glass is transparent in the visible light, but not necessarily
(33:22):
in other wavelengths, and at higher energies more things are
transparent because the photons sort of have enough energy to
get through them. So if you could see it higher
energy photons, then you could see through more stuff. You
could have X ray vision, for example, if you could
see X rays, which in the end are just higher
energy photons, then you could literally see through people. You
(33:45):
could see whether they have a broken bone. You can
detect all sorts of different fascinating things about the world.
So absolutely, yes, the world would look very different if
we could see in lower, higher frequencies of light. And
don't forget that this information is out there all around you.
There's a huge, uge amount of information about the world
that you are missing because you just do not have
the sensors to pick it up. And while we're on
(34:06):
the topic of listener questions about light, I want to
tackle one more. Here's another amazing question. What happens when too. Obviously,
wavelengths light winds contact each other, where do they go?
In the fourth donation, So what if you have a
photon out there at five nanometers and a photon at
seven d animeters and you shoot them at each other,
then what's going to happen? I think that's sort of
(34:27):
the source of the question. Well, unfortunately, not much, because
photons don't interact with things that don't have electric charge. Remember,
photons are the force carrying boson of the electromagnetic interaction.
So anytime there's a magnet or there's electricity, photons are
the thing that's sort of carrying that information. And electromagnetism
(34:49):
works on things that have electric charges. You only have
electrical forces on things that have positive or negative charges,
even magnets. Magnets are generated by little, tiny spinning charges.
So photons only interact with things that have charges, meaning electrons,
meaning protons meaning positrons. They don't interact with things that
don't have charges like other photons. So mostly what happens
(35:12):
when one photon is in the same space as another
photon is nothing. They just pass right through each other. Now,
very occasionally you can have photons interacting with other photons. Remember,
photons are quantum particles, so they're always doing crazy stuff,
and every photon is occasionally turning into a matter antimatter
pair like an electron, as a kind o positron. This
(35:33):
happens very briefly and then it goes back to being
a photon, but it might do that at the same
moment that another photon coming the other direction does the
same thing, and then you'll have an electron an oppositron
from the first photon and an electronpositon from the second photons,
and those the guys can interact. So photons can interact,
(35:54):
but not directly. They have to sort of transform into
other particles briefly, which can then interact. We call that
light by light scattering, and it's actually quite a fascinating experiment.
All right, So we've dug into the physics of light.
We talked about what light is. It's just wiggling electromagnetic fields.
We talked about how light has different frequencies and how
(36:14):
those frequencies translate into color, and the complicated things that
are going on inside your eyeball so that you perceive
those different colors, and the amazing question of whether you
could ever describe your red to somebody else where that
you could think up the new color in somebody's mind.
I love all these questions, and I'm never gonna stop
trying to think up a new color. I'll align my
(36:34):
bed tonight, closing my eyes and trying to imagine a new,
weird kind of colors. Can't be orange, it can't be purple,
it can't be a new kind of green. It's got
to be something totally new. So thanks for tuning in
and listen to me talk and explain all about the
physics of light. Hope you enjoyed that. And if you
have a topic you'd like to hear us talk about,
please send it in to questions at Daniel and Jorge
(36:58):
dot com. Yeah, if you still have a question after
listening to all these explanations, please drop us a line.
We'd love to hear from you. You can find us
at Facebook, Twitter, and Instagram at Daniel and Jorge that's
one word, or email us at Feedback at Daniel and
(37:21):
Jorge dot com. Thanks for listening and remember that Daniel
and Jorge Explain the Universe is a production of I
Heart Radio. For more podcast from my Heart Radio, visit
the i heart radio, app, Apple podcasts, or wherever you
listen to your favorite shows.
When I was a kid, I was fascinated by color,
and in particular, there was one question which had me
up late at night thinking about it, which was this,
can you think up a new color? Now? If you've
seen a rainbow, then you know that the whole spectrum
of visible light is reflected there. You have all the reds,
all the oranges, all the greens, all the yellows, all
(00:30):
the way up to the blues and the violets, and
you can see they're all of the colors that you
can perceive. And of course it makes you wonder about color,
How does color work? What is it really? And it
also connects to some deep questions about philosophy, not just physics.
For example, many people have wondered the color that I'm
seeing red, how do I know that other people are
(00:50):
seeing the same color? Right? Maybe the thing that I
see is red somebody else sees as blue. Fascinating question
in philosophy. But there's another question there, which is can
you think up a new color? If these colors that
I'm seeing are just perceptions in my mind? Is my
brain capable of coming up with a new color? Can
(01:11):
I generate in my own head a new experience of color.
I spent many nights thinking about whether it was possible
to concentrate hard enough to come up with a new
color Hie. I'm Daniel. I'm a particle physicist and a
(01:43):
part time podcast host and the co author of the
book We Have No Idea, A Guide to the Unknown Universe,
which takes you on a tour about all the things
we don't understand about the universe. And you're listening to
the podcast Daniel and Jorge Explain the Universe production of
My Heart Radio. My co host Jorge Ham and co
author in that book, can't be here today, so I'm
(02:05):
talking to you on my own about all the amazing
things in the universe. Our podcast tries to find incredible,
mind blowing, really hard to think about things and explain
them to you in a way that you actually understand
and maybe even entertains you along the way. Today on
the program, we're gonna be walking a fine line between
physics and philosophy, because there's a deep connection between these fields.
(02:28):
Sometimes in physics we discover something that reveals the truth
of the universe, and that truth can make us feel
differently about our relationship with life and the universe and
how everything works. This, of course is true when we're
talking about the beginning of the universe and how it
all came to be in its potential future end, but
also about how we perceive the universe, the very everyday thing,
(02:50):
and one of the most tangible ways we have to
perceive the universe, of course, is with light, and specifically
with color. Color is so physical, it's so tangible, it's
so such an intense experience. But what is it? What
does physics have to say about color? And so that's
the topic we're gonna be tackling on today's podcast. What
(03:14):
is the physics of color? And there's lots of different
aspects to this question. How many colors are there? Why
do we see things in different colors? Why some objects
different colors than other objects? Has it all work? And
there's a great history here of physicists diving into color.
Even Isaac Newton did some of his original best work
with lenses and optics and prisms, and he studied spreading
(03:37):
of white light into the rainbow. And in the early
part of this century, color was a big clue that
helped us understand quantum mechanics. People saw all sorts of
weird patterns that they didn't understand, and it took some
clever brains and some interesting experiments to untangle it. Now,
everybody has some understanding of color. Everybody has some experience
(03:57):
of color. Well, some people out there might be color blind.
But does everybody understand color? Do people know how color works?
Why we see things different color, why some things reflect
blue and other things reflect green? Do people really understand
what color is? So to get a sense of the
general level of knowledge of color, I walked around this
time in Aspen, Colorado, and I asked people what they
(04:19):
knew about color and light and why different things were
different colors. Listen to what they have to say, but
first think to yourself. Do you understand color? Do you
understand light? Do you understand why things are different colors?
Can you imagine a new color in your mind? Think
about those things as you listen to these answers. I
couldn't tell you that one something about the light, but
(04:41):
I don't know reflecting light. I don't know the spectrum
from the sun. There's infrared colors. We can't see the colors.
We can see the spectrum of the colors and the
light causing, uh, what you see for colors? I don't
know physical light is a certain wavelength. Your I C
(05:03):
is visible light and has more to do it's the
light bouncing after the objects. I also don't all. I'm sorry,
it's something to do with the light with our eyes
the color of white. So you're probably hear in those
answers that there's definitely some understanding of light and wavelengths
and color, and that there's definitely some physics to it. Right.
(05:25):
People understand that behind color is a lot of physics,
and that's great because we're gonna dig into all of
that physics today. But there's not a lot of understanding
for why different things are different colors. Why is this
bench blue, why is the grass green? All of these things?
How does that work? On a sort of microscopic level.
One of my favorite things about physics is that we
(05:45):
can take the macroscopic universe, the one that we experience,
and take it apart and explain it in terms of
microscopic stuff. We understand the difference, for example, between frozen
water and liquid water in terms of the motion of
the little articles inside, and everything we are experiencing around
us is in the end, just an emergent phenomenon of
(06:06):
these microscopic events, And so we'd like to understand basically
everything around us in terms of the microscopic principles. Right,
what is really happening on the tiniest level that makes
something green or makes something else red. And by the
end of today's podcast, I hope you'll have a solid
understanding of why things are different colors. So let's dig
into it first. What is light and what is color? Well,
(06:29):
let's begin with light. Light, of course, is just electromagnetic radiation.
We talked about this on the podcast fairly often. You
go all the way down from radio waves up to
gamma rays and X rays. All of these things are
just electromagnetic fields that are wiggling. That's why they can
get to you across the vast distances of space. It's
not like sound, where you have the air that's shaking.
(06:51):
Light is the waving of electromagnetic fields. And electromagnetic fields
are a property of space itself, like all the quantum
fields that we talked about on another podcast, Every element
of space has the possibility to have light in it,
or electrons in it, or any of the other quantum fields.
So when a photon passes through space, what that really
(07:11):
means is the electromagnetic fields in that space are oscillating,
and so all kinds of light are just electromagnetic radiation.
Right in the middle of the spectrum is visible light
at about a few hundred nanometers and it's no different
from the light at higher energies and lower energies, except
of course, for that energy. So the properties that a
(07:31):
photon has that you need to understand are just its energy. Now,
its energy is very closely connected to its frequency. The
more energy the photon has, the faster it wiggles, and
the faster it wiggles, the shorter its wavelength. All these
photons have the same speed. They all travel the speed
of light, but they have different amounts of energy per photon.
(07:53):
And every photon you can translate its energy directly into
its frequency, and its frequency directly into its wavelength. It's
really just one piece of information expressed in different ways.
I want to talk a little bit more about that,
but first let's take a quick break. So photons, of course,
(08:21):
are quantum mechanical particles. We've talked on this podcast many
times about how they can be seen as particles, they
can be seen as waves, and it's true, and you
should think about them as quantum mechanical, but only in
the sense that you can only have a certain integer
number of photons. Like you turn on your laser. You
can have one photon, or two photons, or three photons
or seventy four photons. You can't have one and a
(08:44):
half photons. You can't have two point seven two photons.
That's where the quantum mechanics comes in. It's it's a
discrete number of photons. But this other property of photons,
the energy of a photon equivalent again to its frequency
and therefore wavelength that can have any value. A given
photon can have any amount of energy, from very very low,
(09:07):
making it like a radio wave, to very very high,
making an X ray or a gamma ray. We'll talk
later about how photons are generated, and there are some
objects that can only generate photons of certain energy, but
in principle of photon can have any energy. What that
means to the context of color is that every individual
photon can have any energy level, which means you could
(09:27):
have any frequency, which means it could have any wavelength,
And of course the wavelength of the photon is connected
to the color. We perceive photons of different wavelength as
having different colors than a four D nimeters. For example,
we perceive things as very very red up but seven
d nanometers we perceive things as very very blue or
(09:47):
very very violent. So there's a close connection between the
wavelength of the photon and the color that we perceive.
But don't be confused. The color is not a property
of a photon. Sure people say a red photon, but
what they mean is that the photon has a certain wavelength.
We perceive it as red, but the redness is inside us.
(10:10):
There's nothing red about the photon. The photon just has
a certain wavelength. So when we're talking about the physics
of color, let's separate what property the photon has and
our perception, our experience of it. Alright, so back to
the photon. You can have any infinite number of different
wavelengths for a photon. What that means is that potentially
(10:33):
it is an infinite number of colors. If every wavelength
corresponds to a color, then there's an infinite number of
colors out there. Can we perceive an infinite number of colors.
Let's talk for a moment about what about how we
perceive color. Imagine this spectrum of different wavelengths from four
d nimeters up to seven hundred nimeters. Yes, there's an
(10:54):
infinite number of different wavelengths you could stick into that spectrum. Right,
it's the real numbers and an infinite number. Just the
same way, there's an infinite number of numbers between one
and two. Right, there's one, one point one, one point one, one,
one point one, seven, etcetera. I could go on literally
forever and name numbers between one and two. In the
(11:15):
same way, there's an infinite number of wavelengths photons can have,
but we are limited in how we can perceive them.
We can't necessarily tell the difference between two slightly different
wavelength photons. We might perceive them the same way. That's
just a matter of resolution. It's like in your camera
has a certain number of pixels, and so something falls
(11:36):
in one pixel, you can't tell where in the pixel
it landed. Did it land in the center, did it
land towards the edge. You can't tell because your camera
has a certain spatial resolution, a certain number of pixels,
and of course the more pixels it has, the better
it at it is at figuring out exactly where those
photons landed. In the same way, your eye is not
(11:56):
capable of distinguishing between every time need a little difference
in wavelengths. Two photons that have almost the same wavelength
will register exactly the same way in your eye. Now,
in your eyeball, there are actually three different kinds of
cells in the back of the eyeball that's see color.
What they do is they respond differently to photons at
different wavelengths. One of them peaks very very low, it's
(12:19):
mostly responsive around four hundred and fifty nanimeters. The second
kind peaks sort of in the middle of the spectrum,
like five hundred fifty nanimeters, and the third kind peaks
a little bit higher, just under six hundred nanimeters. So
I have three kind of cells. Each one is sensitive
to different wavelength photons. So the way that it works
is that the photon hits your eyeball and then some
(12:41):
of these light up. If the photon has a wavelength
which corresponds to the peak of the sensitivity for one
of those cells, it'll light up really strongly, like the
one at four fifty. If you send a photon and
at four hundred fifty nimes right at that one, it's
gonna light up, And the other ones are not going
to light up very strongly, whereas if you send a
photon around six hundred nimes, then the third kind is
(13:02):
going to light up really strongly and the other two
are going to be dimmer. And then your brain takes
that information. It says, the low wavelength one lit up
and the other two didn't, so therefore the light we're
seeing must be low wavelength, or if it gets messages,
it say that only the high wavelength sensor lit up,
and then it knows that the light you're seeing must
(13:22):
be high wavelength. It's not that your your eye specifically
measures the wavelength of any individual photon. What it does
is it asks how much does it light up each
of these three sensors, and then it has to reverse
engineer and estimate what was the wavelength of the light
that hit it. It's sort of like triangulation. Your cell
(13:44):
phone knows where it is because it can talk to
like three different cell phone towers, and it can ask
those towers how far away from you am I? And
if one of the tower says, oh, you're real close,
and the other two say no, you're pretty far, then
your phone knows it's pretty close to one of those towers,
and it can tell exactly where it is because it
has the messages from all three. That's called triangulation. Well,
(14:06):
your eye is doing the same thing with the sensors
in the eyeball. It gets three pieces of information about
the light that's coming in, and each of those gives
it different information, right, information about how close are you
to the wavelength that this sensor is good at seeing,
and then it can use that information to decide what
wavelength of the light actually hits you. So the final
(14:27):
perception is sort of mixed from these three different measurements
we make. And this is why you can build up
any sort of color that humans can experience out of
just three sort of basis colors. You often hear about
the primary colors or red, green, and blue, and any
color the humans perceive can be built up with some
combination of red, green, and blue. And this blew my
(14:50):
mind the first time I thought about it. I thought, Wow,
there's like colors live in some sort of mental, abstract
mathematical space, and red, green, and blue, or like the eye,
the invectors of it, and any color you can imagine
is just a linear combination of those three colors. That
was incredible to me, but it's not actually true that
just encompassed the human experience of color. Remember, there's an
(15:13):
infinite number of colors in the spectrum because it's an
infinite number of wavelengths. What r GB does is it
plays with human responses. We have three ways to measure colors,
and so it triggers those three sensors in different ways
to give you the experience of different colors. The same
way in the case of the cell phone towers, if
you could pack those cell phone towers with different distances,
(15:34):
you could stimulate being any place between those towers in
that same way. All right, So to recap photons have
any arbitrary wavelength, which is which is controlled by the
energy that they carry. And if we imagine the relationship
between wavelength and color, color is part of the human perception.
(15:55):
Color is what we experience. There's nothing red about the photon.
Per se. It has a certain wavelength which your brain
measures your eyeballs like a device for measuring the wavelength
of those colors by using those three different sensors to
triangulate it, and then it gives you the experience of
that color. But because there's nothing in particularly red about
(16:16):
the photon. Where does redness come from? And this is
where physics crosses into the realm of philosophy, or physics
inspires fascinating questions in philosophy. And one of the really
interesting wrinkles here is that not everybody out there has
three color sensors in their eye. There are some folks
(16:38):
out there that have a mutation. They have four kinds
of sensors in their eye. They are called tetra chromats,
and they have an extra way to sense color in
their eye. When I first learned about this, I thought, oh,
does that mean that they have like another color in
their mind? Is this fourth kind of cell that can
detect another element of the spectrum give them a new
(17:01):
kind of experience that I can't have. Is there some
color out there that they can experience that I will
never know? That's not the case. Actually, it's just a
fourth way of sensing the wavelength of the light that
you're seeing. So it gives them better ability to nail
down the wavelength of the light. They don't necessarily see
any new colors. It's like adding a fourth tower to
(17:23):
your triangulation. It gives it helps you separate in cases
where it's hard to tell. It gives you extra information
to tell where that cell phone is, it doesn't necessarily
give you a totally new experience of distance. So tetrachromats
are interesting and fascinating, but they don't necessarily see color
differently than we do. They're just better at it. It's
(17:44):
like if you're measuring the length of something and your
ruler has more little marking, so you can make a
more precise measurement of the length of whatever it is
you're looking at. All right, but back to the sense
of philosophy, perception has to be something in the mind,
because again, there's nothing blue or purple or orange about
the photon. That's something that your brain is doing. And
(18:04):
that's why, of course people wonder, is the read that
I'm experiencing different from the red that you're experiencing. Maybe
the red that I'm experiencing is you're blue. That seems
unlikely because we all sort of like the same kind
of art and the same kind of combinations of colors.
But we don't really know, because we can never really
experience what's in somebody else's mind. And this is a
(18:25):
famous question in philosophy. Can you describe redness? Can you
communicate somehow. Is there any possible way to capture the
experience of redness to convey that to somebody else without
them experiencing your redness? Can you describe redness in other terms?
Or is it unique? Is it? Is it its own
(18:46):
sort of basis concept in the idea structure of your mind.
And there's this famous thought experiment. Say you take a
genius scientist and you put her in a room, and
and the scientists only ever see black and white, and
she can learn all about the world, and she can
learn all about science, but she only ever sees black
and white. There's only black and white. Thinks in the room,
(19:07):
and the TV she's using only has black and white,
so she never sees any color. And this person is
super duper smart. Is there any way that she can
understand color so that when she opens the door and
you let her out of this terrible mind experiment you
can never actually do on people. So when she emerges
into the world and sees color and experiences it for
(19:28):
the first time, she will have already understood. Is there
any way to give her that understanding without the experience.
If so, then it means that color is something that
you can translate into other ideas and convey from mind
to mind. If not, then it means it's something purely internal,
something that cannot be described in any other way, meaning
(19:51):
that we can never know if my red is the
same as your red. So it's a famous unanswered question
and philosophy. But it stimulates in me another question, which
is how many colors are there in our mind? I mean,
if they are just in our mind, If the red
that I'm seeing as I look at this T shirt
right now is something an experience that my brain is
(20:12):
generating for me, then can it also generate other colors?
Obviously you can. You can generate blue, can generate orange,
You can generate purple. Right. It takes some external stimulation
to make that happen. But the generation and the experience
itself is in my mind. It's after the information from
the wavelength has been transformed into some sort of pulse
(20:33):
in my brain, and that's when the experience of purple happens.
So then the question is could I generate a novel
one could I think of? Could I imagine new color
that nobody has ever imagined, or at least that I
have never imagined? You know, say I had never seen
anything green before in my life. I had only ever
seen red and blue. Could I think up green? Could
(20:56):
I envision green in my mind without having ever seen it?
Or even if I've seen red, green and blue, can
I come up with a new color? So I honestly
spent many afternoons as a kid trying to come up
with a new color, and it always ended up something
weird and orange. But I never succeeded, and so to
this day, I still do not know the answer to
that question. So that tells us a little bit about
(21:20):
the physics of color. What is color? How is it
connected to the wavelength of light and electromagnetic radiation, and
how we perceive it and what that means. How you
translate from the photons that are out there in the
universe to our perception of color, which is fascinating, but
it doesn't tell us about what's happening microscopically in stuff.
Why is that shirt blue? In this shirt red? Why
(21:42):
is it generating photons at different colors? So I see
those things, so I experience those We'll dig into all that,
but first let's take a little break. Okay, we're talking
(22:04):
about the physics of color and the experience of color.
And this is something which goes back to the early
nine hundreds when there was a really interesting scientific puzzle
that people were trying to understand, which is that some
gases have color. You've probably experienced this if you've ever
played with like a Bunsen burner and put some weird
stuff in it and you see it, Oh, it glows green.
(22:25):
Or if you put this metal in it, you get
something purple. If you put this metal in it, you
get a red flame. And so fire has different colors.
And remember the fire is just essentially ionized gas. You're
heating something up and it's glowing and emitting photons, and
that's what you're seeing. But back on the day before
we had a really detailed understanding of the quantum mechanics
of it, people were wondering why do different gases have
(22:46):
different colors? And more specifically, there were two things that
people noticed. First of all, they noticed the gases absorbed colors.
So if you're shown, for example, a white light through
a bunch of gas, you measure the wavelength of the
light that came through, you'd notice that the gas absorbed
certain wavelengths, but only certain wavelengths, and it depended on
(23:08):
the gas. Nitrogen would absorb different things than hydrogen would
absorb different things than oxygen. So each gas seemed to
have its own pattern. These little slices of the spectrum
that we get taken out of the white light when
they pass through the gas. So you pass white light
through a gas and it removes a certain little slices
of that spectrum, and it's like a fingerprint. You can
(23:29):
tell what gas is there based on which slices of
the spectrum it takes out. But nobody understood why does
this gas take out that those colors and why does
that gas take out the other colors? And the second
thing is the inverse of that. You took those same
gases and you heated them up, and they would glow,
but they wouldn't glow in every color. They don't glow
(23:51):
white necessarily. They glow in certain colors. And the colors
they glow in match exactly the colors that they would
take out of the spec drum when you pass white
light through them. So, for any particular gas, if you
passed white light through it, it would slice out little
parts of the spectrum. But then if you took that
same gas and you heated it up, it would emit
(24:12):
light and exactly those little wavelengths that it had sliced out.
So something interesting was going on. And before people understood
the microscopic physics of it, there was a lot of
study and just a lot of sort of thought about it.
People measured the wavelengths, of course, very carefully and did
detailed experiments to try to understand it, because data, of
course is the source of insight and much of the science,
(24:33):
and especially in physics, and a lot of mathematicians looked
at those spectrum and they noticed patterns. They noticed that
there was spacing between the wavelengths that the that the
gases would absorb, and they saw these patterns that the
spacing would grow larger and larger and larger, and they're
able to fit mathematical equations to those spacings. Now, they
didn't understand where those equations came from, but they noticed
(24:55):
that they were there. So Ridberg, for example, came up
with this formula, and he had no understanding for what
causes formula could couldn't explain the formula at all, but
it worked perfectly. And that's a great clue because it
tells you what's the mathematical structure and the end physics
is always trying to describe the universe in terms of mathematics.
(25:15):
The stated goal of physics, of course, is to write
down an equation that describes everything in the universe and
then look at that equation and understand from the mathematical
structure of that equation, what do we learn about the
nature of the universe. So math is our language. So
as soon as we can turn a big pile of
data into sort of a compressed mathematical equation, then we
(25:36):
can ask questions about the structure that equation and wonder
why is it this way, what is it? Why is
it that way? And it was Neil's bore that figured
it out when he built his atomic theory, the one
that has little electrons orbiting the center. And of course
that's passe because we don't think these days about electrons
orbiting because they're not classical objects that have paths, and
(25:56):
we'll dig into that in a future episode about quantum mechanics.
In his model, electrons were orbiting the nucleus of the atom,
and they could only have certain energy levels. And what
happened when an electron jumped down an energy level It
had to give up some of that energy, and it
gave up that energy in terms of a photon. So
if an atom has certain restricted energy levels, then the
(26:19):
electron can jump down only certain distances, and those distances
correspond to the energy of the photons that can be
emitted by that atom, and therefore correspond to the wavelength
of the light that you see. So if you take
any particular atom, it has certain energy levels, and if
you heat that up, then the electrons jump up energy levels.
(26:39):
They're absorbing that energy, and then sometimes they jump down,
and when they jump down they give off those photons.
So that explained why certain gases emitted only in certain spectrum,
and every gas has its own particular set of wavelengths
that it can emit in those wavelengths again controlled exactly
by the difference in the energy levels of the electrons
(27:01):
going around the center. And it also, awesomely also explained
the absorption because if you take white light and you
shine it at that gas, it can't absorb any arbitrary photon.
It can only absorb photons that will take the electron
up one energy level, or two energy levels, or three
energy levels. And it's this mathematics that explain that spectrum
(27:25):
that the electrons have to move up or down one
or two or three steps, no half steps, no quarter steps,
no one point to seven steps that determine which photons
the atoms can absorb and can emit. So that helps
us understand sort of the physical basis of why different
things give off different colors, why different things look different colors.
(27:46):
So let's put it all together. You have light from
the sun. Now, life from the sun is in lots
of different frequencies, is a broad spectrum. It peaks in
the yellow or sometimes people say it's a little bit green,
but mostly you have life from the sun all all
over the visible spectrum. And that's not a coincidence that
the sun happens that you give off photons in the
same spectrum that we can see things. Right, our eyes
(28:08):
evolved in the presence of this sun in order to
be able to see photons which were around us. We
can think of it as evenly spread across all of
the wavelength. Now, what happens when that light hits your
red T shirt. Well, when light hits your red T shirt,
it gets reflected off the T shirt, but not entirely.
(28:29):
Some of the colors of that white light get absorbed
by your red T shirt. Why does your red T
shirt absorbed only some colors Because the atoms in your
red T shirt have electrons which can jump up one
energy level and accept photons at just the right wavelength. So,
just like the gas where if you pass white light
through it, it will delete certain wavelengths, your T shirt
(28:52):
will delete a bunch of wavelengths from white light. And
your T shirt is red not because it's absorbed photon
on which are in the red part of the spectrum,
but because it's reflected them. This is a common misperception.
People think white light comes from the sun and your
T shirt is red because it's absorbed the red parts
and reflected everything else. Remember that you are seeing photons
(29:14):
only when they hit your eyeball, and so I see
your shirt is red because your shirt has reflected those
red photons. To me, right, light is something I'm experiencing
based on the photons that are being reflected or emitted
from an object, not some like inherent property that it has.
Something absorbs red photons, It doesn't turn that object red.
(29:36):
To see something as red, you have to see red
photons leaving it, which means they have to reflect from
that object. So something that's blue, for example, absorbs red photons.
Something that's red absorbs blue photons to a little bit
backwards right. Or more specifically, something that looks blue absorbs
photons of every wavelength except for blue. Something that looks green, mean,
(30:00):
absorbs photons of every wavelength except for green. And this
is a model of color we call subtractive color because
you start from the white light, which is every kind
of wavelength, and you remove stuff. When something hits your
blue t shirt, a bunch of photons get absorbed, right,
they get removed, So we call that subtractive color. There's
another way to think of color, and that's additive color.
(30:23):
Instead of its starting from full white light and talking
about the color you perceive. If you start from nothing,
you start from blackness, for example a computer monitor, as
opposed to a piece of paper. Start from a computer monitor,
then you can add light to make various mixtures. But
but it's a little bit complicated. The two different ways
of thinking about light are fundamentally equivalent in the end.
(30:45):
But if you design something, for example, on your computer monitor,
and then you print it out on a white piece
of paper, it might look a little bit different from
you expected. So those are you out there who are
artists know all the details about the difference between subtractive
color models and additive color models. All right, So we've
been talking about color and photons, and now I think
we have a pretty good understanding of the physics of it. Remember,
(31:07):
photons have certain wavelength which corresponds to their energy, and
they're just flying around the universe having a certain energy
per photon. The experience of color is something that happens
inside our brain, is the interpretation of signals along the
optic nerve that comes from the eyeball. The eyeball has
done its best to measure the wavelength of the light
that's hitting it. But the experience of color is something internal,
(31:29):
something in the mind, something that philosophers can probe and
physicists can wonder about. But it also makes us wonder
what it's like to experience the world, and whether we
could see the world differently if we had different kinds
of eyeballs. So we've got a great question from a
listener which I want to actually answer right now. Here's
the question. Hi, Daniel Hire, This word that looks pretty
(31:51):
good and sharp in the visible spectrum, of light. But
what would it look like if you could only see
lowware or eh our frequency is off flight? Would a
low frequency world be all transparent? Thank you, what a
great question. I love imagining alternative universes where we had
different kinds of eyeballs or different kinds of experiences. So
(32:14):
it's an interesting question and actually one that you could
answer yourself because we have technology for this. For example,
night vision goggles do this sort of frequency shift, and
they'll let you see light that's out there that your
eyeballs cannot measure. They'll let you see at night because
there are actually photons flying around just that your eyes
cannot see them, in the same way that like infrared cameras.
(32:38):
Infrared cameras see photons that are have too long a wavelength,
wavelength that your eyes cannot see, but that are out there.
And so in the infrared, the world certainly does look different.
Have you seen the Predator movies, for example, where you've
seen any sort of military action movie, you know that
infrared you can see people's heat, you can tell what's
(32:58):
hot and what's not because things glow in the infrared
when they're hot, and so you can definitely have a
different experience of the world if you could see a
different wavelengths, and yes, different things would be transparent and
different things would be opaque because the opacity of something
and its transparency is a function of its wavelength. Right,
glass is transparent in the visible light, but not necessarily
(33:22):
in other wavelengths, and at higher energies more things are
transparent because the photons sort of have enough energy to
get through them. So if you could see it higher
energy photons, then you could see through more stuff. You
could have X ray vision, for example, if you could
see X rays, which in the end are just higher
energy photons, then you could literally see through people. You
(33:45):
could see whether they have a broken bone. You can
detect all sorts of different fascinating things about the world.
So absolutely, yes, the world would look very different if
we could see in lower, higher frequencies of light. And
don't forget that this information is out there all around you.
There's a huge, uge amount of information about the world
that you are missing because you just do not have
the sensors to pick it up. And while we're on
(34:06):
the topic of listener questions about light, I want to
tackle one more. Here's another amazing question. What happens when too. Obviously,
wavelengths light winds contact each other, where do they go?
In the fourth donation, So what if you have a
photon out there at five nanometers and a photon at
seven d animeters and you shoot them at each other,
then what's going to happen? I think that's sort of
(34:27):
the source of the question. Well, unfortunately, not much, because
photons don't interact with things that don't have electric charge. Remember,
photons are the force carrying boson of the electromagnetic interaction.
So anytime there's a magnet or there's electricity, photons are
the thing that's sort of carrying that information. And electromagnetism
(34:49):
works on things that have electric charges. You only have
electrical forces on things that have positive or negative charges,
even magnets. Magnets are generated by little, tiny spinning charges.
So photons only interact with things that have charges, meaning electrons,
meaning protons meaning positrons. They don't interact with things that
don't have charges like other photons. So mostly what happens
(35:12):
when one photon is in the same space as another
photon is nothing. They just pass right through each other. Now,
very occasionally you can have photons interacting with other photons. Remember,
photons are quantum particles, so they're always doing crazy stuff,
and every photon is occasionally turning into a matter antimatter
pair like an electron, as a kind o positron. This
(35:33):
happens very briefly and then it goes back to being
a photon, but it might do that at the same
moment that another photon coming the other direction does the
same thing, and then you'll have an electron an oppositron
from the first photon and an electronpositon from the second photons,
and those the guys can interact. So photons can interact,
(35:54):
but not directly. They have to sort of transform into
other particles briefly, which can then interact. We call that
light by light scattering, and it's actually quite a fascinating experiment.
All right, So we've dug into the physics of light.
We talked about what light is. It's just wiggling electromagnetic fields.
We talked about how light has different frequencies and how
(36:14):
those frequencies translate into color, and the complicated things that
are going on inside your eyeball so that you perceive
those different colors, and the amazing question of whether you
could ever describe your red to somebody else where that
you could think up the new color in somebody's mind.
I love all these questions, and I'm never gonna stop
trying to think up a new color. I'll align my
(36:34):
bed tonight, closing my eyes and trying to imagine a new,
weird kind of colors. Can't be orange, it can't be purple,
it can't be a new kind of green. It's got
to be something totally new. So thanks for tuning in
and listen to me talk and explain all about the
physics of light. Hope you enjoyed that. And if you
have a topic you'd like to hear us talk about,
please send it in to questions at Daniel and Jorge
(36:58):
dot com. Yeah, if you still have a question after
listening to all these explanations, please drop us a line.
We'd love to hear from you. You can find us
at Facebook, Twitter, and Instagram at Daniel and Jorge that's
one word, or email us at Feedback at Daniel and
(37:21):
Jorge dot com. Thanks for listening and remember that Daniel
and Jorge Explain the Universe is a production of I
Heart Radio. For more podcast from my Heart Radio, visit
the i heart radio, app, Apple podcasts, or wherever you
listen to your favorite shows.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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