Question

Why focal length of concave lens increase when immersed in water? Explain?

Answer 1

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.