Electrons emitted with negligible speed from an electron gun are accelerated through a potential difference V along the x-axis. These electrons emerge from a narrow hole into a uniform magnetic field B directed along this axis. However. Some of the electrons emerging from the hole make slightly divergent angles as shown in fig., show that these paraxial electrons are refocused on the x-axis at a distance `sqrt(8pi^2 mV)/sqrt(eB^2)`.

Explanation

Give magnetic field = B,
Potential different = V, ltbr. Mass of electron = m,
Charge = q
Let electtric field be `E' = (V)/r`
`v^(2) = 2 xx a xx s` [u = 0)`
Time taken by particle to cover the are A since the accin is along 'y' axis
it travel along - axis in uniform velocity
Therform `S = v xx t` .