Re: Why does light travel slower thru glass or water than thru a vacuum?

Hi Frank,

That's a very good question! The speed of light itself doesn't change as it goes through different media; instead, the decrease in the "travel time" is a reflection of the interactions of light with matter. As I was preparing my answer, I did a little search at my two favorite sites and found answers there to your question that were much more coherent than mine was shaping up to be. So, I hope you don't mind if I defer to their answers.

The first interpretation is from MadSci Admin extraordinaire John Link and was originally presented here; the abridged version follows:

Another of your statements is "Light will slow down when entering an
optically dense medium."  Actually this is not quite true.  While it
is true that we measure a lower AVERAGE speed of the light through a
medium, the propagation of light through the medium, between atoms, is
actually at the normal vacuum speed of light, c.  What happens is that
the light moves at speed c between atoms, but photons are "absorbed"
by the medium's atoms.  By "absorption" I mean that the energy of the
photon causes an electron of the atom to be kicked to a higher energy
level, and the photon ceases to exist.  Then, after a very small time
delay, the electron goes back to its original (usually ground state)
energy and "emits" a photon of the same energy (and thus same
frequency and thus same wavelength) as the original "absorbed" photon.
(In fluorescent materials the energy of the photon is downshifted, but
I am talking here of "elastic", or non-energy shifting, absorptions.)
It is this very small time delay which makes us measure the average
"speed of light" through the medium as slower than the vacuum speed of
light.  But, again, between atoms the light does travel at the speed
c.

When a light wave enters matter, the light wave's electric field
causes charged particles in the matter to accelerate back and
forth. That's because an electric field exerts forces on charged
particles. The light wave gives up some of its energy to these charged
particles and is partially absorbed in the process. However, the
charged particles don't retain the light's energy very long. They are
accelerating and accelerating charged particles emit electromagnetic
waves. In fact, they reemit the very same light wave that they
absorbed moments earlier. Overall, the light wave is partially
absorbed and then reemitted by each electrically charged particle it
encounters, so that the light continues on its way as though nothing
had happened.

      However, something has happened--the light wave has been delayed
ever so slightly. This absorption and reemission process holds the
light wave back so that it travels at less than its full speed. If the
charged particles in the matter are few and far between, this slowing
effect is almost insignificant. But in dense materials such as glass
or diamond, the light wave can be slowed substantially.

      Actually, higher frequency violet light is slowed more than
lower frequency red light because violet light is more effectively
absorbed and reemitted by the atoms in most transparent
materials. That's because when a high frequency light wave encounters
the electrons in an atom, the jiggling motion is so rapid and the
electrons' motions are so small that the electrons never reach the
boundaries of the atom. As a result, those electrons are able to
jiggle back and forth as though they were free electrons and they do a
good job of slowing the light wave down. But when a low frequency
light wave encounters the electrons in an atom, the jiggling motion is
slower and the electrons' motions are so large that they quickly reach
the boundaries of the atom. As a result, those electrons aren't able
to jiggle back and forth as far as they should and they don't slow the
light wave down as well.


      When a light wave passes through matter, the charged particles
in that matter do respond--the light wave contains an electric field
that pushes on electrically charged particles. But how a particular
charged particle responds to the light wave depends on the frequency
of the light wave and on the quantum states available to the charged
particle. While the charged particle will begin to vibrate back and
forth at the light wave's frequency and will begin to take energy from
the light wave, the charged particle can only retain this energy
permanently if doing so will promote it to another permanent quantum
state. Since light energy comes in discrete quanta known as photons
and the energy of a photon depends on the light's frequency, it's
quite possible that the charged particle will be unable to absorb the
light permanently. In that case, the charged particle will soon reemit
the light.

      In effect, the charged particle "plays" with the photon of
light, trying to see if it can absorb that photon. As it plays, the
charged particle begins to shift into a new quantum state--a "virtual"
state. This virtual state may or may not be permanently allowed. If it
is, it's called a real state and the charged particle may remain in it
indefinitely. In that case, the charged particle can truly absorb the
photon and may never reemit it at all. But if the virtual state turns
out not to be a permanently allowed quantum state, the charged
particle can't remain in it long and must quickly return to its
original state. In doing so, this charged particle reemits the photon
it was playing with. The closer the photon is to one that it can
absorb permanently, meaning the closer the virtual quantum state is to
one of the real quantum states, the longer the charged particle can
play with the photon before recognizing that it must give the photon
up.

      A colored material is one in which the charged particles can
permanently absorb certain photons of visible light.  Because this
material only absorbs certain photons of light, it separates the
components of white light and gives that material a colored
appearance.

      A transparent material is one in which the charged particles
can't permanently absorb any photons of visible light. While these
charged particles all try to absorb the visible light photons, they
find that there are no permanent quantum states available to them when
they do. Instead, they play with the photons briefly and then let them
continue on their way. This playing process slows the light down. In
general blue light slows down more than red light in a transparent
material because blue light photons contain more energy than red light
photons. The charged particles in the transparent material do have
real permanent states available to them, but to reach those states,
the charged particles would have to absorb high energy photons of
ultraviolet light. While blue photons don't have as much energy as
ultraviolet photons, they have more energy than red photons do. As a
result, the charged particles in a transparent material can play with
a blue photon longer than they can play with a red photon--the virtual
state produced by a blue photon is closer to the real states than is
the virtual state produced by a red photon. Because of this effect,
the speed at which blue light passes through a transparent material is
significantly less than the speed at which red light passes through
that material.

      Finally, about quantum states: you can think of the real states
of one of these charged particles the way you think about the possible
pitches of a guitar string. While you can jiggle the guitar string
back and forth at any frequency you like with your fingers, it will
only vibrate naturally at certain specific frequencies. You can hear
these frequencies by plucking the string. If you whistle at the string
and choose one of these specific frequencies for your pitch, you can
set the string vibrating. In effect, the string is absorbing the sound
wave from your whistle. But if you whistle at some other frequency,
the string will only play briefly with your sound wave and then send
it on its way. The string playing with your sound waves is just like a
charged particle in a transparent material playing with a light
wave. The physics of these two situations is remarkably similar.

I hope that helps! I know the last response was probably a little more detail than you were looking for but I think it's better to give too much information than too little. But if you still have any questions, please feel free to drop me an email.

Enjoy!

Rick.