Which of the following is required for obtaining the spectrum of infrared rays:
(A) Glass
(B) Quartz prism
(C) Rock salt prism
(D) Water

Hint Spectrum of rays is obtained due to diffraction of light rays when these rays pass through objects which allow passing rays through it and entering and leaving the surface of the object is not parallel. Spectrum of infrared rays are obtained when infrared rays have passed through an object which allows them to pass through it.

Complete step by step solution
As we know, we obtained the spectrum of infrared rays by passing these rays through an object which is transparent for infrared rays (means infrared rays can pass through it).
Here, water and glass absorb the infrared rays due to which rays are unable to pass through the glass and water. Quartz prisms also absorb the infrared due to which we never get spectrum using it.
Only rock salt prism allows infrared rays to pass through it, and we can get a spectrum of infrared rays using rock salt prism only.

Hence, the correct answer is option C.

Note Here we know that to obtain spectrum entering and leaving the surface of an object is not parallel and in case of water and normal it is also a reason not to get spectrum. With the help of quartz prism, we can get a spectrum of white light rays, but not infrared rays because the wavelength of infrared rays are greater in comparison of white light, and it gets absorbed by quartz glass.