Pretend a person can walk only at a certain pace – no faster, no slower. If you time her uninterrupted walk across a room of known length, you can calculate her walking speed. If, however, she stops momentarily along the way to greet others in the room, the extra time spent in her brief interactions gives an average speed across the room that is less than her walking speed. How is this like light passing through glass? In what way is it not?

Explanation:

If a person is set to walk on a constant speed regardless of the situations then if the person walks a certain distance with no interruptions in an observed time then her speed can be calculated.

When the same person walking through the room momentarily stops to introduce herself then the average speed of the of the person slows down as it happens with the light wave when passes through glass which is an optically denser medium than the air, but the light wave does not stop anywhere in the medium.

It can be more relevant to the person's speed when she walks wading through the water or the person walks through the sand then the person feels resistance in the sand or in water which reduces her overall speed.

a.   mass number from atomic number