Give reason for the following:
(i) Infrared radiations are used for photography in fog.
(ii) Infrared radiations are used for signals during 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 ultraviolet light.
(vi) Ultraviolet bulbs have a quartz envelope instead of glass.