Question

define focus and focal length ?
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Answer 1

Answer:

Principle focus is the the line passing through the focus and centre of curvature to the pole of the mirror,and the focal length is the distance between the focus and the pole of the mirror.

Answer 2

Answer:

The distance of the principal focus from the center of the lens is known as focal length. The first principal focus of a lens is the position of the object on the principal axis of the lens, for which the image is formed at infinity.

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