Question

Write a note on electrostatic focusing.

Answer 1

Electrostatic deflection is the method of aligning the path of charged particles by applying the electric field between the deflecting plates.

Diagram above represents the electrostatic focusing.A and B are two co-axial cylinders with potentials V1 and V2 such that V2>V1.R is the equipotential ring placed between A and B.
Working :
(1) Consider electron beam 1:
It will remain normal to all the equipotential surfaces and hence it is simply accelerated without any deviation of the path.
(2) Consider electron beam 2:
It will have following 2 effects:
(a) On the L.H.S off R:The parallel component of F_P will move the electron towards right while the normal component F_N will move the electron downwards by applying Fleming’s left hand rule at point C.
(b) On the R.H.S off R: F_P and F_N will move the electron towards right and towards up respectively by applying Fleming’s left hand rule at point D.
(3) Consider electron beam 3:
It’s path will be as shown with same case as case(2).
The focal length can be changed by varying V₁ and V₂ 
(4) Bethe’s laws is also followed in electrostatic focusing  :

`(v_2)/(v_1)  = (sin θ_i) /(sinθ_r)`
(5)Electrostatic focusing is used for accelerating and focusing electron beams.