Give reason for the following:
(i) Infrared radiations are used for photography in fog.
(ii) Infrared radiations are used for signals during the war.
(iii) The photographic darkrooms are provided with infrared lamps.
(iv) A rock salt prism is used instead of a glass prism to obtain the infrared spectrum.
(v) A quartz prism is required for obtaining the spectrum of the ultraviolet light.
(vi) Ultraviolet bulbs have a quartz envelope instead of glass.
(i) Infrared radiations are used for photography in fog because they are not much scattered, and so they can penetrate appreciably through it.
(ii) Infrared radiations are used for signals during the war because they are not visible and they are not absorbed much in the medium.
(iii) The photographic darkrooms are provided with infrared lamps because they provide some visibility without affecting the photographic film.
(iv) A rock salt prism is used instead of a glass prism to obtain the infrared spectrum because the rock salt prism does not absorb the infrared radiations, whereas a glass prism absorbs them.
(v) A quartz prism is required for obtaining the spectrum of the ultraviolet light because ultraviolet radiations can pass through quartz whereas ordinary glass absorbs them.
(vi) Ultraviolet bulbs have a quartz envelope instead of glass because ultraviolet radiations are not absorbed by quartz whereas ordinary glass absorbs the ultraviolet light.