Principal Focus and Secondary Foci. Differences. Class 10, Class 12 Physics. JEE MAIN, NEET

What is the principal focal point of a lens? Let us have a convex lens. This red line is principal axis of this lens. Now, if a bunch of parallel rays, parallel to the principal axis of a lens, get refracted through it, you could get a bunch of converging rays. And these converging rays eventually meet at a single point on the principal axis. If you move the source of the rays, keeping them parallel to the principal axis, the point of convergence does not move. It remained fixed on the principal axis. This fixed point on the principal axis is called the principal focal point and is named as FP. Now, if another bunch of parallel rays, but not parallel to the principal axis of the lens, pass through it, the refracted rays will converge to another point. Not at the principal focal point. Not on the principal axis. This point of convergence is called a secondary focal point and denoted as F1. If you change the angle of incidence, the refracted rays will converge to another point. This is also a secondary focal point. This is point F2. Again, a further change in angle of incidence will make the refracted rays to converge to a new point. This is another secondary focal point. Thus, you see that for each angle of incidence, there is a unique secondary focal point. Now, if you study thoroughly, you could find that the principal focal point and all other secondary focal points lie on same plane. This plane is called the focal plane. A focal plane contains all focal points. Principal focal point and secondary focal points. And it is perpendicular to the principal axis of the lens. So, what is principal focal point? If a beam of parallel rays, parallel to the principal axis of the lens get refracted through it, then the point of convergence of these refracted rays is called principal focal point of the lens. And what is a secondary focal point? If a beam of parallel rays, not parallel to the principal axis of the lens, get refracted through it, then the point of convergence of these refracted rays is called the secondary focal point of the lens. Now, what are the four differences between principal focal point and secondary focal point? First difference. Principal focal point lies on the principal axis of the lens. But, a secondary focal point not. It can be at any point on the Second difference. Distance from the optical center of a lens to its principal focal point is the shortest. It is called the principal focal length of the lens. And denoted with the letter small f. But distance from the optical center of the lens to a secondary focal point is always greater than the principal focal length. Third difference. A lens has only two principal focal points. Either side of the lens. But it has innumerable secondary focal points. All of them lie on these focal planes of the lens. Fourth difference. Principal focal length is an important quantity in calculation of image details. While secondary focal length is not so important regarding the same. Here are the all four differences in written form. You can verify them. Have you cleared your concepts on this topic? Leave a comment if you have any doubt. Like this video. And subscribe to this channel to get more such content.