Question

A narrow beam PQ of white light is passing through a glass prism ABC as shown in the diagram.

Trace it on your answer sheet and show the path of the emergent beam as observed on the screen DE.

(i) Write the name and cause of the phenomenon observed.

(ii) Where else in nature is this phenomenon observed?

(iii) Based on this observation state the conclusion which can be drawn about the constituents of white light.

Answer 1

(i) Dispersion of light

Cause: The dispersion of white light occurs because colors of white light travel at different speeds through glass prism. Different colours undergo different deviations on passing through prism.

(ii) Rainbow is the example of dispersion of light observed in nature.

It is caused due to dispersion of sunlight by water droplets in the atmosphere. It always forms in the direction opposite to the sun.

(iii) White light is a mixture of seven colours. The sequence of colours given by the prism is Violet, Indigo, Blue, Green, Yellow, Orange and Red. VIBGYOR is the acronym for this sequence. The red light bends the least and violet light bends the most.

Answer 2

(i) The phenomenon of the splitting up of the white light into its constituent colours is called dispersion of light.

Dispersion of light is caused because different constituent colours of light offer different refractive indexes to the material of the prism.

(ii) The formation of the rainbow is caused by the dispersion of the white sunlight into its constituent colours.

(iii) Based on the dispersion of white light into its constituent colours, we can conclude that-

(a) the white light consists of seven colours.

(b) the violet light suffers maximum deviation and the red light suffers minimum deviation.