Question

A timekeeper in a 100m run is advised to start the stopwatch by seeing the smoke from the starter’s gun and not by hearing the sound of fire. Why?

Answer 1

Hint: The reason for starting the stopwatch by seeing the smoke from the gun instead of waiting for the sound of the gun can be found out by comparing the speed of light and speed of sound in air.

Complete step by step answer:
It is given that a timekeeper in a $100\,m$ run is advised to start a stopwatch by seeing the smoke from the starter’s gun and not by hearing the sound of fire.
This is because light travels faster than sound. When the starter fires a gun, the smoke from the gun can be seen at the same instant but the sound will reach us a bit delayed. We know that sound is a longitudinal wave. It requires a material medium to travel. The sound travels through a medium by vibrating the particles in the medium. The speed of sound in air is calculated by the equation
${V_s} = \sqrt {\dfrac{{\gamma P}}{d}} $
Where, $\gamma $ is a constant having value $1.4$ in air, $P$ is the pressure of air and d is the density.
In air, $P = {10^5}\,N/{m^2}$ and density of air is $d = 1.29\,kg/{m^3}$
So, the velocity of sound in air is
$ \Rightarrow {V_s} = \sqrt {\dfrac{{1.4 \times {{10}^5}}}{{1.29}}} $
$ \Rightarrow {V_s} = 330\,m/s$
This is the velocity of sound in air.
Whereas light is a transverse wave. It can travel even through vacuum. It does not require a material medium to travel. The velocity of light in air is $3 \times {10^8}\,m/s$ .
We can see that this value of velocity of light is much greater than the velocity of sound.
So, if you start measuring at the instant when we see the smoke then we will get the accurate measurement of time. If we wait for the sound of the gun to start the stopwatch then the time measured will not be accurate since there is a small delay in hearing the sound.
Hence it is always advisable to start the stopwatch by seeing the smoke from the gun rather than waiting for the sound to be heard.

Note:
Remember that the speed of sound in air is affected by various factors such as density, pressure, humidity, wind etc. The maximum speed of sound in air is always much less than that of light. The same reason applies for seeing the lightning flash first and hearing the thunder later.