Focus and focal length of lens/first focus/second focus/class 12 Physics/#lens #lenses #focallength

The principal focus of a convex lens is a point on its principal axis to which light rays parallel to the principal axis converge after passing through the lens.

The focal length is the distance of the principle focus from the optical center of a lens.

For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens. For a converging lens (for example a convex lens), the focal length is positive and is the distance at which a beam of collimated light will be focused to a single spot. For a diverging lens (for example a concave lens), the focal length is negative and is the distance to the point from which a collimated beam appears to be diverging after passing through the lens.

First focusSecond focus of lensFocus of convex and concave lens