Class 10 Science

Dispersion of Light

Segregation of white light into different component colours (when light passes throug a prism) is called dispersion of light.

These notes are based on the chapter Human Eye and Colourful World from class 10 science NCERT book and CBSE syllabus.

  • Refraction of light through prism
  • Formation of rainbow

Refraction of Light Through a Prism

scattering light prism

Prism is a transparent optical element which refracts light. An optical object to be defined as prism must have at least two faces with an angle between them. Triangular prism is the most common type of prism. It has a triangular base and rectangular sides. Thus, a traingular prism has two triangular faces and three rectangular faces. Prisms also come in other shapes.

When a ray of light enters the prism, it bends towards the normal; because light is entering from a rarer medium to a denser medium. Similarly, when the light emerges from the prism, it follows the laws of refraction of light. Due to the angle of the prism and due to different wavelengths of different components of white light; the emergent ray gets segregated into different colours. Finally, a colourful band of seven colours is obtained. This phenomenon is called dispersion of white light by the prism.

If you will keep two prisms together, you will get a white beam of light on the screen. In this case, dispersed light from the first prism is combined by the second prism to produce white light.

Formation of Rainbow

You will seldom find a person who is not mesmerised at the sight of a rainbow. Rainbow is formed just after the rain. You may have noticed that rainbow is always formed in a direction that is opposite to the sun's position in the sky.

Following is the scientific explanation of formation of rainbow.

Diagram explaining formation of rainbow

Raindrops work like a prism. When white light enters a raindrop, it experiences refraction, dispersion and internal reflection inside the raindrop. The emergent light experiences refraction of light. As a result, rainbow is formed against the backdrop of sky. The whole process happens in following steps:

  1. Refraction of light happens when ray of light enters raindrop from air.
  2. Dispersion of light happens inside the raindrop.
  3. Internal reflection happens when dispersed light hits the opposite boundary of raindrop. This makes the rays of light to move back in opposite direction.
  4. Refraction of light happens once again when dispersed light comes out of the raindrop. Thus, the rainbow is visible in a direction that is opposite to the direction of the sun in sky.

Thus, rainbow is formed because of combined effect of refraction of light, dispersion of light and internal reflection of light.

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