Question

How does the length of the convex lens increase when we put them in water?

Answer 1

The focal length increases in water because there is less difference in index of refraction between glass and water than between glass and air. Since the relative refractive index of glass with respect to water is less than that of glass with respect to air, the bending of light at the interface of glass water is lesser and hence a parallel beam of light falling on the lens is bent less, to meet at point on the principal axis and hence the lens has more focal length in water than in air.