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Explain the concept of electrostatic focusing in electron optics.
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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 $V_1$ and $V_2$ such that $V_2$>$V_1$. R is the equipotential ring placed between A and B .

WORKING

(1) consider electron beam 1

It will remain normal to all 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

Fp and Fn 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 (2)

The focal length can be changed by varying $V_1$ and $V_2$.

(4) Bethe's Law is also followed in electrostatic focusing

$v_2/v_1$ = $sin@_i/sin@_r$

(5) Electrostatic focusing is used for accelerating and focusing electron beams