Question

The purpose of electron gun in a cathode ray tube is:
A. to change the electric signal to the visual signal
B. to give out a fine energetic beam of electrons
C. to deflect the electron beam
D. to produce electrons

Answer 1

A cathode ray tube is a mechanically constructed device in which a finely curated beam of electrons is focussed and made to hit another electrode, which can then be made to hit a photosensitive/electrosensitive screen or analysed further to get different properties like current, voltage etc.

Complete answer:
-Cathode ray tubes mainly consist of an electron gun, a screen, two or more electrodes and a glass casing and terminals to connect to the input and output sources.
-In the electron gun assembly, the parts include a heater, filament, an emitting electrode which acts as the cathode and two anodes, for accelerating and focussing the electron beam.
-It works by first heating the filament, which transfers this heat to the cathode. This leads to the excitation of the cathode, which then emits a large number of electrons.
-The accelerating anode creates a potential difference large enough for the electrons to get attracted towards it. These electrons are then focussed by the focussing anode to provide a fine beam of highly energetic electrons. This is the entire process which goes on inside an electron gun.
-Hence, the function of an electron gun is to give out a fine energetic beam of electrons .
So, the correct option is B.

Note:
A common misconception is that an electron gun is a device which only emits electrons. However, this is wrong, and the full function of the gun is to produce a fine beam of energetic electrons.
Note that the accelerating electrode must have a potential which is significantly larger than the potential of the cathode for the proper functioning of the gun.
Also note that vacuum conditions must be maintained inside the tube, otherwise the electrons will hit the atoms present in air, thus, losing their functionality.