Chapter 27, Problem 39A

If you fire a ball through a pile of sand, it will slow down in the sand and emerge at less than its initial speed. But when light shines on a pane of glass, even though it slows down inside, its speed upon emerging is the same as its initial speed. Explain.

To determine

To explain that the speed of light remain same after passing through glass while the speed of ball decrease after passing through a pile of sand.

Answer to Problem 39A

When the ball strikes to a pile of sand its momentum decreases and hence speed decreases. Hence emerge at less than its initial speed. But since the light has no mass and has constant speed in vacuum, it emerges in vacuum at same speed after passing through glass medium.

Explanation of Solution

Introduction:

Light is an electromagnetic wave which has zero mass. The speed of light remain constant in vacuum. But in a medium such as glass, water etc. light travels through it with slower speed.

When the ball strikes to a pile of sand its momentum decreases and hence speed decreases. Beside it, the ball takes more mass dragging with itself during its motion through pile of sand which also decreases its speed and hence emerge at less than its initial speed.

But since the light has no mass and has constant speed in vacuum, it emerges in vacuum at same speed after passing through glass medium.

Conclusion:

Thus, explained that the speed of light remain same after passing through glass while the speed of ball decrease after passing through a pile of sand