>   What happens when the photon "hits" a refractive object? Why does it
"bend"
> at the surface of the object? Why does it seem to go slower through
the object?

>   I have heard this explanation that the photon is "absorbed" and
another
> photon (apparently a different one) is emitted. However, I have never
> understood this explanation. Even in very solid matter atoms and
molecules are
> extremely far apart from each other, which means that there's quite a
lot of
> space between them; how come every and each one of the incoming
photons are
> absorbed and none goes through the space between the atoms without
never
> colliding with them?

Though this sort of thing was never my strong point, I'll have a go...

While the spaces between the nuclei in a solid might be very large,
electrons "fill" these spaces by virtue of their exceedingly high speeds
and quantum mechanical nature.  Electrons interact primarily through the
EM force here, and as a photon is the mediating particle of the EM
force, the two very readily interact.  It's probably wise to recall the
wave/particle duality of light, since the wavelength of visible light is
large compared to the spacing between atoms (a few 100x larger).  This
wavelength can be thought of as the "size" of the photon in this case I
think, and so combining the large "size" of photons and the ease of
interaction with electrons, you have a large "cross-section" for the
interaction.  The larger the cross-section, the more likely the
reaction.

> And what causes the change in direction at the surface
> of the object? What causes the change in direction inside the object
if the
> density of the object is not constant? Why does the photon _always_
change
> its path to the exact same direction?

Good question.  The answer is probably buried deep in some obscure
equation somewhere, and even then could probably get summed up as "it
just does" :)  In other words, I have no idea.  A related question I
guess is how exactly a mirror works.

> How, for example, electrons can travel
> in water faster than photons can (ie. why don't electrons collide in
the same
> way as photons; electrons are a lot bigger)?

IIRC electrons have a "drift velocity" of a few cm per second in a
conducting metal.  This is the actual speed at which they move through
the metal when conducting electricity.  However, as Tim says, they shove
each other along, and as soon as one starts moving, all of the others in
the circuit are pushed into moving.

Finally I should note that a photon NEVER travels slower than c.  ALL
apparent slowing of light is due to absorption/re-emission.  While this
gives rise to a "speed of light in material x", the photons themselves
are still moving at c.  Other particles can and do exceed this speed,
however, giving rise to a phenomenon known as Cerenkov radiation.  A
high energy particle will produce a "bow wave" when it travels faster
than the apparent speed of light in a material, visible as a cone of
light (which tends to be blue).  This is the reason why the water in
cooling ponds (is that the term?) of nuclear reactors, where they leave
the spent fuel to cool off for a while, glows blue.  It's the Cerenkov
radiation resulting from all the nuclear decay products whizzing through
the water.

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