Question

Why are red coloured lights used in traffic signals to stop the vehicles?
(A) Minimum wavelength and less scatter.
(B) Maximum wavelength and more scatter.
(C) Maximum wavelength and less scatter.
(D) Less wavelength and less scatter.

Answer 1

Hint : To answer this question, we need to compare the wavelength of the red light to that of other lights lying in the visible region of the EM spectrum. Then using Rayleigh’s scattering we can get the correct answer.

Complete step by step answer
For stopping the vehicles moving on the rod, we use the light as a traffic signal. And this light must be in the visible region of the EM spectrum so that it can be perceived by the human eye. We know that the colours of lights which fall in the visible region are; violet, blue, green, yellow, orange, and red. The range of the wavelengths corresponding to the visible region is from $ 400nm $ of violet light to $ 700nm $ of red light. So we see that the wavelength of the violet light is the least and that of the red light is the maximum in the visible region.
Now, in the path from the traffic light to the observer’s eye, lies the atmospheric particles. The atmospheric particles scatter the light which travels through them. This reduces the visibility of the signal. So we have to select the light which is least scattered by the atmospheric particles. This scattering is related to the wavelength of the light by the law of Rayleigh’s scattering as
 $ S \propto \dfrac{1}{{{\lambda ^4}}} $
So the scattering is inversely proportional to the fourth power of wavelength of the light.
As the wavelength of red light is maximum, so it is least scattered by the atmospheric particles. Therefore it is used in traffic signals.
Hence, the correct answer is option C.

Note
The scattering of light is responsible for many interesting phenomena occurring in nature. For instance, the sky appears blue because the blue light has a low wavelength and hence is more scattered by the atmospheric particles.