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Question

If you fire a ball through a pile of sand it will slow down and emerge at less than its initial speed. But when light slows down inside a pane of glass it reemerges at the initial air speed. WHY?

Answer 1

It isn't the same light. The light that emerges at the other end is light that has been transmitted by those last few molecules of glass that absorbed and retransmitted light from within the glass. If this were not so, if light were not hitting, absorbing and retransmitting, you would have the equivalent of 'holey' glass, the same situation as if the glass had holes and the light just went by all the glass molecules. In the second case, with the holes, the light would still have the initial air speed. It's all this absorption and retransmission that gives light it's speed in any particular medium and why light speed is fastest in a vacuum.

Answer 2

The difference is that light has no mass.