Question

Find the optical power and the focal lengths 

(a) of a thin glass lens in liquid with refractive index n₀ = 1.7 if its optical power in air is Φ₀ = —5.0 D; 

(b) of a thin symmetrical biconvex glass lens, with air on one side and water on the other side, if the optical power of that lens in air is Φ₀ = +10 D.

Answer 1

(a) Optical power of a thin lens o f R.I. n in a medium with R.I. n₀ is given by :

The second focal length, is given by 

 where n is the R.I. of the medium in which it is placed.

(b) Optical power of a thin lens of RI. n placed in a medium of RI. n₀ is given by : 

           (A)

For a biconvex lens placed in air medium from Eqn. (A)

        .....(1)

where R is the radius of each curve surface of the lens 

Optical power of a spherical refractive surface is given by :

                   .....(B)

For the rear surface of the lens which divides air and glass medium

Similarly for the front surface which divides water and glass medium

          .......(3)

Hence the optical power of the given optical system