Question

A lens is made of Flint glass of refractive index 1.5. When the lens is immersed in a liquid of refractive index 1.25 the focal length -
1. increases to a factor of 1.25
2. increases to a factor of 2.5
3. increases to a factor of 1.2
4. decreases to a factor of 1.2

Answer 1

the focal length is increasing to factor of 2.5

Answer 2

4. decreases to a factor of 1.2
because power is inversley propertional to focal lenght