ISC (Arts) Class 12CISCE
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Dispersion by a Prism

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Dispersion of white light by a glass prism

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REFRACTION OF LIGHT THROUGH A PRISM:-

-For parallel refracting surfaces, as in a glass slab, the emergent ray is parallel to the incident ray. However, it is slightly displaced laterally.

-Consider a triangular glass prism. It has two triangular bases and three rectangular lateral surfaces. These surfaces are inclined to each other. The angle between its two lateral faces is called the angle of the prism.

Here PE is the incident ray, EF is the refracted ray and FS is the emergent ray. You may note that a ray of light is entering from air to glass at the first surface AB. The light ray on refraction has bent towards the normal. At the second surface AC, the light ray has entered from glass to air. Hence it has bent away from normal. Compare the angle of incidence and the angle of refraction at each refracting surface of the prism. The peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation. In this case ∠D is the angle of deviation.

DISPERSION OF WHITE LIGHT BY A GLASS PRISM:-

The prism has probably split the incident white light into a band of colours. Note the colours that appear at the two ends of the colour band. The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red. The acronym VIBGYOR will help you to remember the sequence of colours. The band of the coloured components of a light beam is called its spectrum. You might not be able to see all the colours separately. Yet something makes each colour distinct from the other. The splitting of light into its component colours is called dispersion.

WHITE LIGHT IS DISPERSED INTO ITS SEVEN COLOURS.

A rainbow is a natural spectrum appearing in the sky after a rain shower

It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere. A rainbow is always formed in a direction opposite to that of the Sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop.

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