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 (mu = 1.33).

Answer

30 cm

Explanation

a. `1/f=(mu-1)[1/R_1-1/R_2]`
`1/f=0.6[1/20-1/30]`
`=0.6/(1060xx10)`
`f=100cm`
b. `1/f=[mu_(lens)/mu_(medium)-1][1/R_1-1/R_2]`
`=(1.60-1.33-1)[1/60]`
`28/(133xx60)~=1/300`
`f=30 cm`