Page 30 - cbse-class-10-light-reflection-and-refraction

The principal axis is the straight line that connects the centers of curvature of a
lens's two curved surfaces.

Principal Foci

Rays of light can pass through the lens in any direction, so there will be two
principal foci on either side of the lens, which are referred to as the first and second
principal foci of a lens, respectively.

First Principal Focus (F )
1

It is a point on the lens's principal axis where light rays starting from it (convex
lens) or appearing to meet at the point (concave lens) become parallel to the lens's
principal axis after refraction from the two surfaces of the lens.

The distance between the optic center and the first focus is referred to as the lens's
first focal length (f ).
1

Second Principal Focus (F )
2

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