Why is glass transparent? - Mark Miodownik

Take a look out your window,

put on your glasses if you wear them.

You might want to grab a pair of binoculars, too,

or a magnifying lens.

Now, what do you see?

Well, whatever it is,

it's not the multiple layers of glass

right in front of you.

But have you ever wondered

how something so solid can be so invisible?

To understand that,

we have to understand what glass actually is,

and where it comes from.

It all begins in the Earth's crust,

where the two most common elements are

silicon and oxygen.

These react together to form silicon dioxide,

whose molecules arrange themselves

into a regular crystalline form known as quartz.

Quartz is commonly found in sand,

where it often makes up most of the grains

and is the main ingredient in most type of glass.

Of course, you probably noticed that glass

isn't made of multiple tiny bits of quartz,

and for good reason.

For one thing, the edges of the rigidly formed grains

and smaller defects within the crystal structure

reflect and disperse light that hits them.

But when the quartz is heated high enough

the extra energy makes the molecules vibrate

until they break the bonds holding them together

and become a flowing liquid,

the same way that ice melts into water.

Unlike water, though, liquid silicon dioxide

does not reform into a crystal solid when it cools.

Instead, as the molecules lose energy,

they are less and less able

to move into an ordered position,

and the result is what is called an amorphous solid.

A solid material with the chaotic structure of a liquid,

which allows the molecules to freely fill in any gaps.

This makes the surface of glass uniform

on a microscopic level,

allowing light to strike it

without being scattered in different directions.

But this still doesn't explain

why light is able to pass through glass

rather than being absorbed as with most solids.

For that, we need to go all the way down

to the subatomic level.

You may know that an atom consists of a nucleus

with electrons orbiting around it,

but you may be surprised to know

that it's mostly empty space.

In fact, if an atom were the size of a sports stadium,

the nucleus would be like a single pea in the center,

while the electrons would be like grains of sand

in the outer seats.

That should leave plenty of space

for light to pass through

without hitting any of these particles.

So the real question is not

why is glass transparent,

but why aren't all materials transparent?

The answer has to do with the different energy levels

that electrons in an atom can have.

Think of these as different rows of seats

in the stadium stands.

An electron is initially assigned to sit in a certain row,

but it could jump to a better row,

if it only had the energy.

As luck would have it,

absorbing one of those light photons

passing through the atom can provide

just the energy the electron needs.

But there's a catch.

The energy from the photon

has to be the right amount

to get an electron to the next row.

Otherwise, it will just let the photon pass by,

and it just so happens that in glass,

the rows are so far apart

that a photon of visible light

can't provide enough energy for an electron

to jump between them.

Photons from ultraviolet light, on the other hand,

give just the right amount of energy,

and are absorbed,

which is why you can't get a suntan through glass.

This amazing property of being both

solid and transparent has given glass many uses

throughout the centuries.

From windows that let in light

while keeping out the elements,

to lenses that allow us to see both

the vast worlds beyond our planet,

and the tiny ones right around us.

It is hard to imagine

modern civilization without glass.

And yet for such an important material

we rarely think about glass and its impact.

It is precisely because the most important

and useful quality of glass is

being featureless and invisible

that we often forget that it's even there.