'Circle of least confusion' refers to which one of the following defects occurring in images formed by mirrors or lenses?

A plano-convex lens of refractive index ($$\mu_1^{\prime}$$ fits exactly into a plano-concave lens of refractive index $$\mu_2$$. Their plane surface are parallel to each other. 'R' is the radius of curvature of the curved surface of the lenses. The focal length of the combination is

The magnifying power of a refracting type of astronomical telescope is '$$\mathrm{m}$$'. If focal length of eyepiece is doubled then the magnifying power will become

A ray of light travels from air to water to glass and again from glass to air. Refractive index of water w.r.t. air is 'X', glass w.r.t. water is 'Y' and air w.r.t. glass is 'Z'. Which one of the following is correct?