Question

Alternate inverted prisms are placed in series. Choose correct options.
A. Even number of prisms does not produce a spectrum
B. Odd number of prisms produce a spectrum
C. More the number of prisms, less is the intensity of emergent ray/ spectrum
D. All of the above

Answer 1

Hint: When white light passes through a prism, it is split up into a spectrum of seven colours. However, if two alternately inverted prisms are kept in series, the split light from the first prism joins up again and comes out as white light from the second prism.

Complete step by step answer:
When white light is made to pass through a prism, it splits up into its component colours forming a spectrum. This phenomenon is known as the dispersion of light by a prism. However, if two alternately inverted prisms are kept in series, the split light that comes out from the first prism again combines to form white light in the second prism.
This happens because two alternately inverted prisms can be imagined to join together and form a rectangular glass slab in shape. Since, we know that a rectangular glass slab does not disperse white light, but only shifts it laterally, hence, a laterally shifted emergent ray comes out from the second prism.
Keeping a third alternately inverted prism would again split the light emerging from the second prism. Hence, it can be extended that an even number of prisms will not produce a spectrum while an odd number will do.
Also, each time at the interface of air and glass of the prism, some of the light is absorbed or reflected while the rest is refracted. Therefore, obviously there is a loss of intensity in the refracted light. As the number of prisms are increased, so does the number of interfaces and hence, the intensity of the refracted light keeps on decreasing. Hence, more the number of prisms, less is the intensity of the emergent ray/ spectrum.

Hence, the correct option is D) all of the above.

Note: Students must keep in mind that this phenomenon occurs only when the prisms kept are in alternately inverted fashion. Even if two consecutive prisms are in the same orientation, then the whole answer will change. Then it is possible that an odd number of prisms will not produce a spectrum while an even number of prisms will. Students must be careful of this point while analyzing the question.