Question

If you fire a bullet through a tree, it will slow down inside the tree and emerge at a speed that is less than the speed at which it entered. Does light, then, similarly slow down when it passes through glass and also emerge at a lower speed? Defend your answer.

Answer 1

I thought I had answered this earlier. Even though light has some particle-like properties, its propagation speed is governed by a "wave equation" of electromagnetism, and it depends only upon the medium it is travelling through. The same is true of sound. Light slows down by about 30% when going through glass, but speeds up again to c = 3*10^8 m/s after emerging out the other side into air or vacuum.