Question

In producing a pure spectrum, the incident light is passed through a narrow slit placed in the focal plane of an achromatic lens because a narrow slit
(A) Produce less diffraction
(B) Increase intensity
(C) Allows only one color at a time
(D) Allows a more parallel beam when it passes through the lens.

Answer 1

Hint:The purity and efficient spectrum is an essential factor. In order to obtain a better slit, narrow slits are used. This is to improve the spectrum.

Complete step by step answer:
The narrow slits are preferred for a high resolution spectrum. The narrower the slit, the resolution will be better. The angular split aperture when we observe from the collimator lens will determine the purity of the spectrum. So the collimator lens would be of enough focal length. By using this, the loss of light can be reduced.
If the slit is wide, then the area through which the incident light passing will be more. Then the light passing through this wide area will overlap and thus the spectrum will not be pure. So for a better spectrum a narrow slit should be placed in the focal plane of an achromatic lens. Thus the area will be small and the incident light passing through the lens will be more parallel.

The answer is option C.

Note: We have to note also that the narrow slits can cause the loss of light. This is because of the small area of the slit and due to the diffraction at the narrower slit.