Explain the functioning of a photocell. Give its two uses.
A photocell is a practically useful device based on the photoelectric effect. It enables us to change light energy into electrical energy. Now-a-days photocells that work with ‘invisible lights’. like infrared radiations, ultraviolet rays, etc., are also available.
The early photocells were of the vacuum tube variety. Such a photocell consisted of an evacuated ‘bulb’ (made of glass for ordinary visible light and, of ‘quartz’ for ultraviolet radiation) which ‘contained’ a suitably supported and appropriately sensitive metal plate (to act as the cathode) of a semi-cylindrical shape.
The ‘metal plate’ chosen was such that its threshold frequency was well below the frequencies of light’ to be used with the photocell. This was achieved by coating the metal surface with a suitable photo sensitive coating like potassium oxide, cesium oxide or rubidium oxide, etc. The evacuated bulb also had a thin rod of platinum or tungsten to serve as the anode. When light was made to fall on the ‘cathode’. there was an emission of photoelectrons from its surface. These photoelectrons were attracted towards the anode (maintained at a positive potential with respect to the cathode) and hence photoelectric current flowed in the external part of the cathode—anode circuit. We are thus, able to get an electric current (electrical energy flow) from light energy.
What are the uses of a Photocell.?
(I) Photocells are used in counting and automatic control systems.
(ii) They can be used to control furnace temperatures and to study astronomical phenomenon like
temperature and spectra of Stars.
A photocell is a practically useful device based on the photoelectric effect. It enables us to change light energy into electrical energy. Now-a-days photocells that work with ‘invisible lights’. like infrared radiations, ultraviolet rays, etc., are also available.
The early photocells were of the vacuum tube variety. Such a photocell consisted of an evacuated ‘bulb’ (made of glass for ordinary visible light and, of ‘quartz’ for ultraviolet radiation) which ‘contained’ a suitably supported and appropriately sensitive metal plate (to act as the cathode) of a semi-cylindrical shape.
The ‘metal plate’ chosen was such that its threshold frequency was well below the frequencies of light’ to be used with the photocell. This was achieved by coating the metal surface with a suitable photo sensitive coating like potassium oxide, cesium oxide or rubidium oxide, etc. The evacuated bulb also had a thin rod of platinum or tungsten to serve as the anode. When light was made to fall on the ‘cathode’. there was an emission of photoelectrons from its surface. These photoelectrons were attracted towards the anode (maintained at a positive potential with respect to the cathode) and hence photoelectric current flowed in the external part of the cathode—anode circuit. We are thus, able to get an electric current (electrical energy flow) from light energy.
What are the uses of a Photocell.?
(I) Photocells are used in counting and automatic control systems.
(ii) They can be used to control furnace temperatures and to study astronomical phenomenon like
temperature and spectra of Stars.
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