Question

The radii of curvature of a lens are + 20 cm and + 30 cm. The material of the lens has a refracting index 1.6. Find the focal length of the lens (a) if it is placed in air, and (b) if it is placed in water (μ = 1.33).

Answer 1

20*30*16/10

60*16=960

Answer 2

Given :

R1=+20cm

R2=+30cm

μg=1.6

a) If placed in air:

1/f= (μg-1)[1/R1-1/R2]

1/f=(1.6-1)[1/20-1/30)

f= 600/6=100cm

b) If placed in water:

1/f= (uw-1)[1/R1-1/R2]

=(1.6/1.33 -1)(1/20-1/30)

f=300cm