Zahid Iftikhar said:
Do you want me to click on the link for you ? What is unclear in the explanation
Electrostatic Focusing System
Electrostatic Focusing
An electrostatic focusing system is shown in figure. Electrostatic lens consists of three anodes, with the middle anode at a lower potential than the other two electrodes.
In figure two anodes and its electrostatic lines and equipotential surfaces are shown. A pd is kept between these two electrodes so that an electric field is generated between them. Spreading of electric field is caused because of repulsion between electric lines. If equipotential lines are drawn, as shown in figure, they would bulge at the centre of the two anodes. As we know that electrons move in a direction opposite to that of electric field lines and equipotential surfaces are perpendicular to the electric field lines so force on the electron is exerted in the direction normal to the equipotential surface.
Electrons entering at the centre line of the two anodes experience no force but electrons displaced from the centre line experience a force normal to the direction of equipotential surface and deflect, as shown in figure. An equi potential surface is shown, in which an electron with velocity V1 and at an angle θ. to the normal of equipotential surface enters and experiences a force in a direction normal to the equipotential surface. Thus the velocity of the electron increases to V2. This force on the electron is exerted in the direction normal to equipotential surface so only the normal component of electron velocity V1N increases to V2N and the tangential component of velocity V1T remains the same.
From figure
V1T = V1 sin θ1,
V2T = V2 sin θ2
But V1T = V2T
V1 sin θ1 = V2 sin θ2
Or V2/V1 = sin θ1 / sin θ2
From the above expression it is obvious that equipotential surface acts as a concave lens in geometrical optics. That is why, this focusing system is named as an electrostatic lens. Now if we go back and refer figure, it can be seen that because of middle anode to a lower potential, electron beam coming from the cathode and passing through the first concave electrostatic lens tends to become more aligned with the axis of CRT and when it enters at the second concave electrostatic lens, formed between two anodes at different potentials, it is focused at the phosphor screen. Focal length of the electrostatic lens can lie adjusted by varying potential of middle anode with respect to other two anodes. Thus electron beam can be made to focus at the screen very precisely.
