Questions and Answers of ICSE Class 8

ICSE Class 8 Answered

Why does the white light split on the first refracting face of prism

Asked by banerjee.smita1 | 11 Oct, 2018, 07:05: PM

Expert Answer

- A prism is made up of a material like glass whose refractive index is greater than 1.

- Now, when a ray of light is incident on the prism, it gets refracted because its speed decreases in the glass.

- However, speed also depends on the wavelength of the light.

- Hence, different coloured lights are deviated through different angles as each colour is associated with different speeds, and hence, different refractive index.

- This is what causes dispersion of light. White light splits into seven colours because of dispersion.

Newton's experiment for dispersion of white light:

- Newton concluded that white light consists of a mixture of seven different colours.

- Each colour corresponds to a small range of wavelength.

- Thus, white light consists of a very large number of wavelengths, i.e. it is polychromatic in nature.

- The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion.

- When white light is incident on the first surface of a prism and enters glass, light of different colours due to different speeds in glass gets refracted through different angles.

- Thus, the dispersion (or splitting) of white light into its constituent colours takes place at the first surface of a prism.

- Hence, the cause of dispersion is the change in speed of light with wavelength (or frequency).

- The deviation towards the base of the prism is different for light of different colours. Red colour is deviated the least, while violet colour is deviated the most.

Answered by Abhijeet Mishra | 12 Oct, 2018, 12:40: PM