One plano-convex and one plano-concave lens of same radius of curvature 'R' but of different materials are joined side by side as shown in the figure. If the refractive index of the material of 1 is $$\mu $$₁ and that of 2 is $$\mu $$₂, then the focal length of the combination is :

A convex lens of focal length 20 cm produces images of the same magnification 2 when an object is kept at two distances x₁ and x₂ (x₁ > x₂) from the lens. The ratio of x₁ and x₂ is :-

A thin convex lens L (refractive index = 1.5) is placed on a plane mirror M. When a pin is placed at A, such that OA = 18 cm, its real inverted image is formed at A itself, as shown in figure. When a liquid of refractive index μ1 is put between the lens and the mirror, The pin has to be moved to A', such that OA' = 27 cm, to get its inverted real image at A' itself. The value of μ₁ will be :-

A concave mirror for face viewing has focal length of 0.4 m. The distance at which you hold the mirror from your face in order to see your image upright with a magnification of 5 is :