The lens maker's formula is useful to design lenses of desired focal lengths using surfaces of suitable radii of curvature. The focal length also depends on the refractive index of the material of the lens and the surrounding medium. The refractive index depends on the wavelength of the light used. The power of a lens is related to its focal length.Answer the following questions based on the above:
The focal length of a concave lens of `mu` = 1.5 is 20 cm in air. It is completely immersed in water of `mu` =4/3 Calculate its focal length in water.