What Is Light?

Is light a particle or a wave? Or both? Or neither? Learn how humans have defined light throughout history -- including our best attempts today -- in this episode of BrainStuff. 

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain stuff from how stuff works, Hey, brain
stuff loin Volga bamb here. Light, in addition to being
a bright patch of sunshine on your window sill, is
a metaphor for enlightenment and exploration, which is a bit
paradoxical for a phenomenon that, even after thousands of years
of inquiries and endless experiments, scientists still can't quite explain.

(00:24):
Is it a particle or a wave or both or neither?
Do we need a new word for it? Your eyes
tell you a lot about the way light behaves. It
travels so fast that it seems instantaneous, about a hundred
and eighty six thousand miles or three thousand kilometers per second.
It blazes through air and space and laser like straight lines,
but it also bounces, reflects, and refracts, and when it

(00:46):
interacts with the right medium, like a camera lens, it
may curve. We know that it's made up of tiny
units that we call photons, and we know that the
term waves can describe its movements, but neither of these
words really encompass lights oddities. In ancient times, the Greeks
used philosophy to attempt to address light's wide range of behaviors.
Perhaps they thought light is actually composed of little bits

(01:08):
of stuff that bounced to and fro. The idea never
really caught on. Then, in the sixteen hundreds, French philosopher
Renee de Cart became convinced that light was essentially a wave,
one that moved through a mysterious substance that he called plenum.
Isaac Newton thought that light was a particle, but he
was at a loss for a way to explain many
of its properties, like the way it refracted and could
be split by a prism from a single beam of

(01:30):
white light into a rainbow of many colors of light.
This was largely before the rise of empirical studies in science,
wherein we attempt to answer questions about the world around
us by designing experiments that demonstrate well how stuff works.
Back in the day, science was a matter of philosophy,
people coming up with ideas about how stuff works and

(01:50):
basically arguing about the idea's merit to be fair. Our
modern microscopes, computers, and other equipment help. Just for example,
light's behavior becomes more evident depending on where you're observing it.
In the vacuum of space, light zips along at the
aforementioned a hundred and eighty six thousand miles or three
thousand kilometers per second. But point a beam of light

(02:12):
at a very dense bit of matter, say a diamond,
and it can slow to only around seventy seven thousand
miles or a hundred twenty four thousand kilometers per second,
much easier to observe relatively. To try to explain in
these are modern times, what light is, Let's first remember
some science basics. Waves are not a thing or a substance.

(02:32):
They're a property of a thing. A wave is a
compressing and stretching of a particular medium, like an ocean
wave that drives towards the shore or the ripple that
spreads out across the surface of a pond. When you
toss in a rock, you can see the waves with
your eyes, feel them with your body, and sometimes when
a sound wave happens in the air, you can hear
them with your ears. Particles, on the other hand, are

(02:54):
not quite so easy to define. A particle can be
a tiny bit of matter, a matter broken down on
into its smallest and most basic units. Water, for example,
is made up of countless particles particles that are affected
by waves. What's really happening when you watch a wave
in the ocean or a ripple in a pond is
that each particle or molecule in this case of water
is being moved, and thus the medium of the ocean

(03:17):
or pond is being compressed and stretched in sequence, and
we see waves. But light, as experiments have proven, also
consists of particles that we call photons that behave like waves.
Let's unpack that. There was a famous nineteenth century double
slit experiment in which researchers beamed light through two slits
and observed the way the light struck a screen behind

(03:39):
the slits. What they saw was that the streams of
light affected each other like two hands splashing water in
the same sink, as if they were waves interfering with
one another. But then in the twentieth century, scientists began
their pioneering explorations into sub atomic particles like neutrons and electrons.
Albert Einstein wondered what would happen if you admitted light
one photon at a time. In the double slit experiment,

(04:03):
what scientists saw dumbfounded them. The single photons went individually
through the slits, but the way that they struck the
screen over time showed the same interference pattern that occurred
with full scale beams of light streaming through both slits.
This behavior can't be explained by the physics we use
to describe particles and waves in the macro world around us.
It's in the realm of quantum mechanics, the physics theories

(04:25):
that describe what goes on at the very smallest sub
atomic levels and which we humans still don't really understand.
So ultimately, if you want to answer the question what
is light, you could call it both a particle and
a wave and you'd be correct. But as for fully
explaining why and how it works, we're still working on it.

(04:48):
Today's episode was written by Nathan Chandler and produced by
Tyler Clang for i Heeart Media and How Stuff Works.
To learn more about the weird behavior of light and
the history of how humans have thought about it, check
out our sister podcast, Daniel mclae explained the universe. Their episode,
Is Light, a Particle or a Wave goes into lots
more details, and of course, for more on this and
lots of other lighthearted topics, visit our home planet How

(05:10):
Stuff Works dot com.