(iii) A convex lens has 20 cm focal length in air. What is focal length in water ?
(Refractive index of air-water = 1.33. Refractive index for ar - glass = 1.5.)

Explanation

iii) For a glass lens in air, `n_(2)=1.5, n_(1)=1. f=+20cm`. Hence, the lens formula gives.
`(1)/(20)=0.5[(1)/(R_(1))-(1)/(R_(2))]`
For the same glass lens in water.
`n_(2)=1.5, n_(1)=1.33`. Therefore,
`(1.33)/(f)=(1.5-1.33)[(1)/(R_(1))-(1)/(R_(2))]`
Combining the above two equations, we find `f=+78.2cm.`