CBSE Class 10 Answered

Focal length is the distance between the pole of a mirror or a lens to the focus of the mirror or the lens.

For a concave mirror and a concave lens the focal length is negative. While the focal length for convex mirror and convex lens is positive. The focal length of a plane mirror is infinity.

The focal length is related to the object distance (u) and image distance (v) as well as the curvature of the reflecting surface.

The focal length for a mirror is given by,

f = R/2, where R is the radius of curvature of the mirror.

1/f = 1/v + 1/u

The focal length of a lens is given by,

1/f = (µ -1)(1/R₁ + 1/R₂)

‘µ’ is the refractive index of the material, R₁ and R₂ are the radii of curvature of the spherical surfaces of the lens.

Also,

1/f = 1/v – 1/u