An excellent question! As materials scientists, we
think quite a bit about optical and electronic
properties of materials. In order to answer your
question, we'll need a few bits from quantum
You may have had some classical (i.e., Newtonian)
physics already during your time in high school.
Classical physics does well to describe the
macroscopic world, i.e., the size of things on the
order of your or me. When you get to small
objects on the order of the size of an atom, you
need to start considering quantum mechanical
effects. One thing classical mechanics tells
you is that energy exists in a continuum. For
example, if you hold a ball above the ground, that
ball has potential energy; you can change the
height of that ball by an infinitely small amount
and the potential energy will change accordingly.
Now if you pretend you can do that with an
electron, quantum mechanics tells you that energy
is now quantized. That is, that electron
can only exist at particular energy levels and
only at those energy levels. This is important for
understanding how light interacts with matter, and
ultimately why glass is transparent.
There are three ways that light interacts with
matter: absorption, transmission, and
reflection. We will focus on absorption and
transmission. Recall that light (or more
generally electromagnetic radiation) is just
another form of energy. Absorption happens
when incident light has enough energy to excite
an electron from its lowest energy, i.e.,
ground, state to a higher energy, i.e., excited,
Note from ScienceLine Moderator: The
diagram has a misspelling. Instead of saying
"excited states", it says "exicted states".
However, if the incoming light does not have
enough energy to excite an electron to an excited
state, that light will simply pass through, i.e.,
transmit. Emission is also possible, but for
materials like glass, you are more likely to get
emission via phonons (i.e., heat) rather
than photons (i.e., light).
In a solid like glass, these energy states form a
set of energy bands and between these
energy bands there may be an energy gap where no
electrons are allowed to transition. For glass,
this energy gap is large. What this means is that
any wavelength of visible light is not sufficient
to excite electrons from the ground state, and
thus transmits. However, UV light is high
enough energy and you would find that glass is
opaque to it.
Fun fact: a similar reason why glass is
transparent is also why glass is such a poor
Another fun fact: these quantized energy
levels are characteristic to each element, and is
in fact used to identify the composition of stars!
Hope this helps!