ICSE Class 10 Physics Chapter 6 Spectrum Revision Notes
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The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion. Rainbow is the natural phenomenon in which dispersion takes place. The cause of dispersion is that sunlight consists of seven constituents (colours namely violet, indigo, blue, green, yellow, orange and red popularly referred to as VIBGYOR) that have a different refractive index with respect to a medium. The dispersion of white light occurs because colours of white light travel at different speeds through the glass prism. The wavelength of each colour is different and this causes the difference in velocity of the corresponding light when passing from one medium to another.
Cause of Dispersion
- The dispersion of white light into its constituent colours takes place at the first surface of a prism.
- The cause of dispersion is the change in the speed of light with a wavelength (or frequency).
- On the second surface of the prism, the only refraction takes place.
- Prism itself produces no colours.
- The colours of a spectrum of white light combine to reproduce white light.
The band of colours obtained on a screen on passing white light through a prism is called spectrum. The prism has split the incident white light into a band of colours. The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red. The band of the coloured components of a light beam is called its spectrum.
Scattering of Light
Scattering of light means to throw light in various random directions. Light is scattered when it falls on various types of suspended particles in its path. Depending on the size of particles, the scattering can be of white sunlight or of the component colours of sunlight.
1. The sky appears blue during a clear cloudless day because the molecules in the air scatter blue light from the sun more than they scatter red light.
2. During sunrise and sunset, the sky appears red and orange because the blue light has been scattered out and away from the line of sight.
Why is the colour of the clear sky blue?
The sky appears blue during a clear cloudless day because the molecules in the air scatter blue light from the sun more than they scatter red light. Scattering of light causes the blue colour of the sky. These are more effective in scattering light of shorter wavelengths at the blue end than the light of longer wavelengths at the red end.
The scattered blue light enters our eyes. If the earth had no atmosphere, there would not have been any scattering. Then, the sky would have looked dark. The sky appears dark to passengers flying at very high altitudes, as scattering is not prominent at such heights.
Colour of the sun at sunrise and sunset
The scattering of light causes the reddish appearance of the Sun at the sunrise or the sunset. Light from the sun near the horizon passes through thicker layers of air and a larger distance in the earth’s atmosphere before reaching our eyes.
Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the Sun.
ICSE Class 10 Physics Chapter 6 Spectrum Revision Notes PDF