What is the difference between focus and principal focus of concave and convex lenses

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Solution

Principal focus is a point (on the principal axis) where parallel incident rays meet after reflecting (in mirrors) or refracting (in lenses). It is a fixed point of particular lens/mirror and depends upon its shape. Principal Focus also determines the focal length of the lens/mirror.

Focus is just a random point where some rays meet, it might be at the principal focus or otherwise. It also need no be placed on the principal axis of the lens/mirror.

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Similar questions

Q. Define the principal focus of a convex lens and that of a concave lens.

Q. What is principal focus for concave lens?

(a) With the help of a labelled diagram explain how a convex lens converges a beam of parallel light rays. Mark the principal axis, optical centre, principal focus and focal length of the convex lens on the diagram.
(b) State whether the convex lens has a real focus or a virtual focus. (c) List some things that convex lens and concave mirror have in common.

(a) With the help of a labelled diagram, explain how a concave lens diverges a beam of parallel light rays. Mark the principal axis, optical centre, principal focus and focal length of the concave lens on the diagram. (b) State whether concave lens has a real focus or a virtual focus. (c) List some things that concave lens and convex mirror have in common.

What is the difference between focus and principal focus.

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