Question

A glass prism is able to produce a spectrum when white light passes through it but a glass slab does not produce any spectrum. Explain why is it so.

Answer 1

In a glass slab the two refracting surfaces are parallel to each other and only lateral displacement occurs and no deviation is there but in case of a prism the two refracting surfaces are inclined at an angle. so each colour of the white under goes a deviation based on the wavelength of the light . so a spectrum is observed. but in case ofa glass slab the net deviation is zero.

Answer 2

Answer:

(a) The splitting up of white light into its constituent colours is called dispersion. The colour sequence is given by the acronym V I B G Y O R – Violet, Indigo, Blue, Green, Yellow, Orange and Red. This colour pattern is called a spectrum.

Dispersion takes place because the speed of light of different colours through a glass prism is different and so, refractive index, therefore, each colour bends (refracts) through different angles with respect to incident ray as they pass through a prism. The red colour has maximum speed in glass prism. So, it is least deviated while the violet colour has minimum speed so its deviation is maximum. Thus, the ray of each colour emerges along different paths and becomes distinct.

(b) For dispersion, the two refracting surfaces must be inclined to each other as in case of prism. In rectangular glass slab, the refracting surfaces are parallel to each other. So, dispersion cannot occur. This is due to fact that the rectangular glass slab can be considered as equivalent of two identical prisms in inverted position placed in an inverted position with respect to each other. The deviation and dispersion produced by the second inverted prism is equal and opposite to that produced by the first prism.

 

Therefore, there will neither be dispersion, nor deviation, Le. second inverted prism recombines the , colour to give a white light parallel to the incident ray again and will undergo only lateral displacement. Hence, rectangular glass slab cannot produce any spectrum.

Explanation: