We often state that the speed of light in vacuum is about 3 × 10 8 m s and the average speed of sound in air is about 330 m s . Why do you think the term 'average' is used to describe the speed of sound in the air but not the speed of light in vacuum?

The distance that the sound waves travel in a given amount of time is called the speed of sound. The average speed of sound in the air is about 330 m s . We know that sound waves are mechanical waves, and they can only travel through matter. The speed of sound through dry air at 20 ° C is about 343 m s . The speed of sound may vary through other matter or other temperatures. Since, the speed of sound may vary according to the temperatures and medium, we use the term average to describe the speed of sound. The speed of the light is experimentally found to be constant. Hence, the term average is not required in describing the speed of light.

A comparison of change in magnitude of physical quantity with respect to time is called rate. For example, the rate of change of velocity is said to be acceleration.