12th physics. NCERT notes

Scattering of Light

As sunlight travels through the Earth’s atmosphere, it gets scattered by a large number of molecules present in the atmosphere.

According to Rayleigh

where Is = intensity of light scattered
λ = wavelength

Also, rays don’t undergo any change in wavelength on scattering.

Application of Scattering of light:

Blue Color of Sky:

As blue has the shorter wavelength than red, therefore; blue color is scattered much more.

White Color of Clouds:

Clouds contain very large particles such as water droplets, ice particles, dust particles etc. As x >> λ.

So, all colors are scattered in equal amount to give white color. Hence clouds appear white.

Sun looks reddish at the time of sunrise & sunset:

At the time of sunrise & sunset, the sun is near the horizon. The rays from the sun have to travel a large part of the atmosphere. As λblue < λred.

Therefore must of the blue light get scattered away & only red light enters our eyes.

Danger signals are red:

Because λred is large and thus is scattered least and can be seen from the maximum distance. That’s why danger signals are red.


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