physics essay question

If you fire a bullet through a block of wood, it will slow down as it passes through the block and emerge at less than its initial speed. If you shine light on a pane of glass, it will slow down as it travels through the glass, but reemerge at light speed. Explain.

That is what it does, all right. Particles and objects with mass, such as bullets, have to slow down when they collide with much slower material (such as the block of wood). Light has no mass. Its speed depends only upon the type of material it is going through. When the light beam emerges from the opposite side of the pane of glass, it is once again in air, where the speed is 3*10^8 m/s. There will be about 8% less energy in the besm, however, because some of the light particles will be reflected at the front and back surfaces.

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