Referring to this, I have a confusion whether the first focus is always left of a convex lens? Can it not be on the right?

The definition of first focus is : “that point on the principal axis of the lens at which if an object is placed, the image would be formed at infinity”.

So such a point can be on both sides of a lens right? so if the rays are incident on a convex lens from the right and after refracting rays go parallel, can we say that the the first focus is on the right of the lens now?


1 Answer 1


First focus is the point where if a object is placed, the image will be formed at infinity.

By definition it isn’t fixed. It depends upon the side at which the object is placed.

So yes, it can be at both sides.


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