Question

(a) Define prism angle and angle of deviation for a prism.

(b) Write the sequence of colours in the spectrum formed by the passage of white light through a prism.
(c) Explain the process of formation of rainbow.

Answer 1

(a) The angle between its two lateral faces is called the angle of the prism or the prism angle.

When the light ray is allowed to pass through the prism, it makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation for the prism.

(b) When white light is allowed to pass through a prism, a spectrum of colours will be formed which is in the sequence as follows:

Red, Orange, Yellow, Green, Blue, Indigo, Violet

(c) 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. Due to the dispersion of light and internal reflection, different colours reach the observer’s eye.