Question

Define focal length and principal axis of a lens

Answer 1

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Focal length

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The distance between the principal focus and optical center of a lens is known as focal length of lens ..

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A line joining the center of curvature of two spherical surfaces forming a lens is called principal axis ..

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Answer 2

hi mate---

Principal axis---
A line passing through the centre of curvature and focus is called principal of Axis.The focus is the point on principal Axis where are all the rays of light ,parallel to principal Axis convert after reflection from lens.
It is denoted by 'P'

Focal length ----

It is the distance between the focal point and the pole of the lens. The focal length of a lens is always found to be half of the radius of the curvature .The focus is the point on principle axis where all the light rays parallel Axis converge after being reflected from the lens.
It is denoted by 'f'

I hope helps you