28. 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 the 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.
Answers
1.Infrared radiation are used in photography at night and also in mist and fog because they have long wavelength. As they are having low frequency the energy associated with them is also low so they do not scatter much and can penetrate appreciably through it
2.Ordinary visible light is scattered by haze but infrared (IR) radiation can penetrate through the haze without being scattered.
3.A darkroom is a workshop used by photographers working with photographic film to make prints and carry out other associated tasks. It is a room that can be made completely dark to allow the processing of the light-sensitive photographic materials, including film and photographic paper
4.Rock salt prism is used instead of glass prism to obtain infrared spectrum because infrared radiations are absorbed by glass but not by rock salt.
6.The envelope of bulb is made of quartz instead of glass because it has a higher melting point temperature than glass. Thus, the filament inside the bulb can be burned at a higher temperature, producing more light.