When a beam of white light is passed through a glass prism, it is split up into a band of colours called spectrum. This is called dispersion of white light. The spectrum of white has the colours violet, indigo, blue, green, yellow, orange and red (VIBGYOR). The red light bends the least and the violet light bends the most.
A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by the dispersion of sunlight by water droplets present in the atmosphere. The water droplets act like small prisms. They refract and disperse the sunlight then reflect it internally and finally refract it again when it comes out of the rain drops. Due to the dispersion of sunlight and internal reflection by the water droplets we see the rainbow colours.
Tyndall effect :When a beam of light passes through a colloidal solution, the path of light becomes visible due to the scattering of light by the colloid particles. This is known as Tyndall effect. The earth’s atmosphere contains air molecules, water droplets, dust, smoke etc. When sunlight passes through the atmosphere the path of the light becomes visible due to the scattering of light by these particles. The colour of the scattered light depends upon the size of the scattering particles. Very fine particles scatter blue light. Larger particles scatter different colours of light.
Why is the colour of the clear sky blue ? The fine particles in the atmosphere have size smaller than the wave length of visible light. They can scatter blue light which has a shorter wave length than red light which has a longer wave length. When sunlight passes through the atmosphere, the fine particles in the atmosphere scatter the blue colour more strongly than the red and so the sky appears blue. If the earth had no atmosphere there would not be any scattering of light and the sky would appear dark. The sky appears dark at very high altitudes.
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