Question

A double concave lens with the refractive index n=1.5 is kept in air .its two spherical surfaces have refractive index r1=15 R2= 75 .THEN find the focal length of the lens

Answer 1

Answer:

Explanation:

The sign convention for quantities is that distances in the direction of light rays are positive. Other way round are negative.

For refraction on the first concave surface we can derive that:

μ₂/v₁ - μ₁/u = (μ₂-μ₁)/R₁

μ₁/v - μ₂/v₁ = - (μ₂ - μ₁)/R₂

Summing up we get 1/v - 1/u = (μ₂₁ - 1) [1/R₁ - 1/R₂]

The quantity on LHS is 1/f.

So Lens maker's equation: 1/f = (μ₂₁ -1) [1/R₁ - 1/R₂]

R₁ = -20 cm , R₂ = +60 cm

1/f = (1.5 - 1) * [-1/20 - 1/60] = - 1/30

f = -30 cm