Question

explain lens maker's formula?

Answer 1

It is a relation that connects focal length of a lens to radii of curvature of the two surfaces of the lens and refractive index of the material of the lens.

The following assumptions are made for the derivation:

- The lens is thin, so that distances measured from the poles of its surfaces can be taken as equal to the distances from the optical centre of the lens.
- The aperture of the lens is small.
- Point object is considered.
- Incident and refracted rays make small angles.

The focal length of a lens in air can be calculated from the lensmaker's equation:

1 ( 1 1 )
--- = (µ - 1) ( ----- - -----)
f ( R1 R2 )

where
f is the focal length of the lens,
µ is the refractive index of the lens material,
R1 is the radius of curvature of the lens surface closest to the light source,
R2 is the radius of curvature of the lens surface farthest from the light source, and