Question

What is meant by scattering of light? Use this phenomenon to explain why the clear sky appears blue or the Sun appears reddish at sunrise.

What is the scattering of light? Use this phenomenon to explain why
(i) the Sun appears reddish at sun-rise, and
(ii) the clear sky appears blue.

Answer 1

The phenomenon in which a part of the light incident on a particle is redirected in different directions is called the scattering of light.

The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. So, they are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end. Thus, when sunlight passes through the atmosphere, the fine particles in the air scatter blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes,
and hence, the clear sky appears blue.

At the time of sunrise and sunset, when the Sun is near the horizon, sunlight travels a greater distance through the atmosphere to reach us. During this, most of the shorter wavelengths present in it are scattered away from our line of sight by the molecules of air and other fine particles in the atmosphere. So, light reaching us directly from the rising or setting Sun consists mainly of the longer wavelength red colour because of which the Sun appears red. Thus, at sunrise and sunset, the Sun as well as the surrounding sky appears red.

Answer 2

Scattering of light is a phenomenon in which light moving in a straight line is forced to deviate in different directions by one or more particles present in the medium.

(i) Light from the sun near the horizon passes through thicker layers of air and a larger distance in the Earth's atmosphere before reaching out to our eyes. Hence during sunrise, most of the blue light and shorter wavelengths are scattered away by the particles present in the earth's atmosphere. There the light reaches our eyes is of longer wavelengths only. The red light, which has a longer wavelength, is least scattered and is able to reach our eyes, Therefore, the Sun appears reddish during sunrise.

(ii)  The sky appears blue because of the scattering of white light takes place in the atmosphere. As we know there are many air particles and other fine particles suspended in the atmosphere have the size smaller than the wavelength of visible light. So, the more effective scattering of light observed for shorter wavelengths (blue light) than longer wavelengths(red light). The red light has a wavelength of about 1.8 times greater than blue light. Thus, when sunlight passes through the atmosphere, the fine particles in the air scatter the blue light (shorter wavelengths) more strongly than red light, that's why the clear sky appears blue.