Why does light slow down when it passes through a medium like water?

I know that the speed of light is constant in a vacuum anywhere, anytime. But why does it slow down when traveling through a medium?

Why does light slow down when it passes through a medium like water?
It goes slower because water has a higher refractive index than the vacuum. The reason for this is that two important quantities called the permeability and permittivity are different in water than the values in free space (a vacuum).
Reply:It has to go around all the fish and other sea creatures.
Reply:The answer depends on whether you look at light as a wave or as a particle (either way is acceptable).





If you look at light as a wave, then it consists of electric and magnetic fields forming and re-forming within the medium. The strengths of these fields, and how strongly they affect each other, is determined by quantities call the permittivity and permissivity of the substance, and that in turn affects (slows down) the speed of the wave.





If you look at light as particles, what you see is that the photons are periodically absorbed and then re-emitted by the atoms in the substance, with a short delay between absorption and re-emission. The photons still traverse the empty spaces _between_ the atoms at "full speed" (c), but because of the starting-and-stopping, it takes them longer to get to the other side of the medium, with the net effect that their average speed is slower than "c".
Reply:Because there is "stuff" in the way. Light moves at a constant speed in a vacuum, a little slower through the air (because of the molecules it has to go through), a little slower yet through water, even slower through oil, and slowest through solids. It has to do with the density of the medium through which it is traveling.

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