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.

Consider a convex lens (or concave lens) of absolute refractive index m _{2} to be placed in a rarer medium of absolute refractive index m _{1}.

Considering the refraction of a point object on the surface XP_{1}Y, the image is formed at I_{1} who is at a distance of V_{1}.

CI_{1}= P_{1}I_{1} = V_{1} (as the lens is thin)

CC_{1} = P_{1}C_{1} = R_{1}

CO = P_{1}O = u

It follows from the refraction due to convex spherical surface XP_{1}Y

The refracted ray from A suffers a second refraction on the surface XP_{2}Y and emerges along BI. Therefore I is the final real image of O.

Here the object distance is

(Note P_{1}P_{2} is very small)

(Final image distance)

Let R_{2} be radius of curvature of second surface of the lens.

It follows from refraction due to concave spherical surface from denser to rarer medium that

Adding (1) & (2)