CBSE Class 10 Answered

A principal focus or focal point is a special focus:

For a lens, or a spherical or parabolic mirror, it is a point onto which collimated light parallel to the axis is focused. Since light can pass through a lens in either direction, a lens has two focal points—one on each side. The distance in air from the lens or mirror's principal plane to the focus is called the focal length.

Elliptical mirrors have two focal points: light that passes through one of these before striking the mirror is reflected such that it passes through the other.

The focus of a hyperbolic mirror is either of two points which have the property that light from one is reflected as if it came from the other.

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