Define the term focal length of a lens. - Vidyadip

Question

Define the term focal length of a lens.

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Answer

Focal length of a lens: Rays of light can pass through the lens in any direction and hence there will be two focal lengths on either side of the lens and they are referred to as the first focal length and the second focal length of a lens:
(i) First focal length: The distance from the optical centre of the lens to its first focal point is called the first focal length $\left(f_1\right)$ of the lens.
(ii) Second focal length: The distance from the optical centre of the lens to its second focal point is called the second focal length $\left( f _2\right)$ of the lens.

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