Question

What is principal focus and focal length of a lens? ​

Answer 1

Answer:

principal focus is the point at which the incident ray intersects after passing parallel to the principal axis....

the distance between pole and focus is known as the focal length...

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Answer 2

Answer:

principal focus is a point where lines parallel to principal axis meet after reflection .

focal length is the distance between pole and principal focus.

f=r/2