The fact that light travels in air about a million times faster than sound, can be used to determine the speed of sound in air.

Experiment —

Choose two places A and B at high altitudes facing facing each other, at a distance d apart (say, about 1 km), in still iair. The distance d is noted. At each place, there is an observer with a gun and a stop watch. First the observer at place A fires the gun, while the observer at place B starts his stop watch immediately on seeing the flash of fire at A and stops it when he hears the sound of fire. The observer at B by his watch thus finds the time interval t_{1} taken by the sound to travel from A to B.

Now the observer at place B fires the gun. The observer at A starts his stop watch when he sees the flash of fire at B and stops when he hears the sound of fire. Thus, the observer A by his watch finds the time interval t_{2} taken by sound to travel from B to A.

The average of the two time intervals is t = t_1+t_2\over 2. This is the time taken by sound to travel the distance d between the places A and B.

The speed of sound is calculated by using the formula —

V = Distance\over Time

In the experiment the speed of sound determined is not very accurate because of the personal error of the two observers and the variation in temperature and humidity of air in between the places A and B.