Write a note on electrostatic focusing.

#### Solution

**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 V_{1} and V_{2 }**(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.