Why Is Glass Transparent?

Glass windows are as solid as the materials that surround them, yet they let light through. Learn the physics of why glass can be so transparent -- and why it took humans thousands of years to create totally clear glass -- in this episode of BrainStuff, based on this article: https://science.howstuffworks.com/question404.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to Brainstuff, a production of iHeartRadio, Hey, brain Stuff,
Lor and Volgevan here. Think about the way of building.
Like a house is constructed. You've got outer walls made
of materials like brick or wood, with openings built in
or cut in for windows, which are frames that hold

(00:22):
panes or sheets of glass. Windows make a home feel bright, warm,
and welcoming because they let sunlight enter. But why should
a glass window be any more transparent than the wood
or brick that surrounds it. After all, both materials are solid,
and both keep out rain, snow, and wind. Yet wood

(00:43):
is opaque and blocks like completely, while glass is transparent
and let's sunshine stream through unimpeded. You may have heard
some people and even some science textbooks try to explain
this by saying that wood is a true solid and
that glass is highly viscous liquid. They go on to
argue that the atoms in glass are spread farther apart

(01:05):
and that these gaps let like squeeze through. They may
even point to the windows of centuries old houses, which
often look wavy and unevenly thick, as evidence that the
windows have flowed over the years, like the slow crawl
of molasses on a cold day. In reality, glass isn't
a liquid at all. It's a special kind of solid

(01:26):
known as an amorphous solid. This is a state of
matter in which the atoms and molecules are locked into place,
but instead of forming neat orderly crystals, they arrange themselves randomly.
As a result, glass is mechanically rigid like a solid,
yet it has the disordered arrangement of molecules like liquids.

(01:47):
Amorphous solids form when a solid substance like silicon dioxide
also known as silica or sand, is melted at high
temperatures and then cooled so fast that it doesn't have
time to form orderly crystal, a process known as quenching.
The panes of glass in old houses aren't wavy and
thicker at the bottom because they're still flowing. They were

(02:09):
made in a time when glass technology wasn't as good
as it is today, so each pain may have had
some rippling in it when it's set and been thicker
on one end than the other. The carpenter would have
put the thicker end on the bottom because it's sturdier
like that. In many ways, glasses are like ceramics and
have all of the same properties durability, strength and brittleness,

(02:31):
high electrical and thermal resistance, and a lack of chemical reactivity. Basically,
glass won't corrode, break down, or discolor, which is why
it's used in so many applications. What's called soda lime glass,
which is the commercial glass that you find in sheet
and plate glass glass, jars and bottles and light bulbs,

(02:53):
has another important property. It's transparent to the range of
electromagnetic wavelengths known as visible light. To understand why, we
have to take a closer look at the atomic structure
of glass and understand what happens when photons, the smallest
particles of light, interact with that structure. Okay, so soda
lime glass is made up of mostly silicon dioxide or silica,

(03:17):
with a little bit of sodium carbonate or soda ash
added for manageability, and calcium carbonate or lime added for
hardness and durability. All of those atoms are arranged in
the amorphous solid body of the glass. Now, think about
the structure of an atom. You've got the atom's nucleus
with any protons and neutrons it has, and then the

(03:38):
electrons around that occupying different energy levels. If an electron
gains energy, it might move to a higher energy level.
If it gives up energy, it might move to a
lower one. In either case, the electron can only gain
or release energy in discrete bundles. Light is made up
of photons. Now, let's that are a photon moving toward

(04:01):
and interacting with a solid object made up of atoms.
One of three things can happen. First scenario, the substance
could absorb the photon. This occurs when the photon gives
up its energy to an electron located in the material.
Armed with this extra energy, the electron is able to
skip to a higher energy level while the photon disappears.

(04:24):
Second scenario, the substance could reflect the photon. To do this,
the photon gives up its energy to the material, but
a photon of identical energy is emitted. Third scenario, the
substance could allow the photon to pass through unchanged, known
as transmission. This happens because the photon doesn't interact with

(04:46):
any electrons and continues its journey until it interacts with
another object. Clear glass falls into this last category. Photons
pass through the material because they don't have sufficient energy
to excite a lo electrons in the glass to a
higher energy level. Physicists sometimes talk about this in terms
of band theory, which says energy levels exist together in

(05:09):
regions known as energy bands. In between those bands are
regions known as band gaps, where energy levels for electrons
don't exist at all. Some materials have larger band gaps
than others. Glass has pretty large band gaps, which means
it's electrons require much more energy before they can skip
from one energy band to another and back again. Photons

(05:33):
of visible light, that is, light with wavelengths from four
hundred to seven hundred nanimeters, corresponding to the colors violet, indigo, blue, green, yellow, orange,
and red, these photons simply don't have enough energy to
cause this skipping in glass. Therefore, photons of visible light
travel right through glass instead of being absorbed or reflected,

(05:56):
thus making glass transparent. At wavelengths smaller than visible light,
photons begin to have enough energy to move the electrons
in glass from one energy band to another. For example,
ultraviolet or UV light, which has a wavelength ranging from
ten to four hundred animeters, cannot pass through most sodo
lime glass, such as the glass and a window pane.

(06:19):
This makes a window, including the window in our hypothetical house,
as opaque to UV light as wood is to visible light,
which is pretty excellent considering the damage that EV light
can do to where eyes and skin. Pure silica glass
would let UV light through. There are other types of
transparent glass too, like lead glass, which is sometimes called

(06:41):
lead crystal and is used in decorative pieces because it
has such high brilliance, especially when cut with lots of facets.
There's also borsilicate glass, which is used for kitchen and
lab equipment because it's much better at withstanding temperature changes
than lead or soda lime glasses. It actually took humans
thousands of years to work out how to make glass clear.

(07:05):
Ancient crafts people in Mesopotamia in Egypt some four thousand
years ago discovered sodoaline glass and cast it to make
objects like beads and hollow containers. However, natural impurities and
the raw materials for glass will cause it to turn colors.
For example, iron oxides will create a blue to green
to yellow to brownish tint. By around the first century

(07:28):
bceeb artisans in Jerusalem invented the technique of blowing glass
very thinly so that it appeared transparent mostly, but it
wasn't until the fourteen hundreds that Venetian glass makers figured
out how to make glass colorless. They achieved this by
carefully controlling the purity of their raw materials and by

(07:49):
adding small amounts of other materials to counteract the tints
caused by iron oxides like antimony, potash, or manganese oxides.
This glass was considered super fancy and also led to
the development of technologies like magnifying lenses. All along this
timeline and up through today, people working with glass have

(08:11):
developed different techniques to color glass on purpose. Modernly, that's
often by adding metals or metal oxides alike copper for
red glass or cobalt for blue. When they do, the
glass will start reflecting those particular wavelengths of light. Today's

(08:32):
episode is based on the article what makes glass Transparent
on how Stuffworks dot Com, written by William Harris. Rain
Stuff is production of by Heart Radio in partnership with
how Stuffworks dot Com, and it's produced by Tyler Klang.
For more podcasts from My Heart Radio, visit the iHeartRadio app,
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