The focal length of a glass $\left(\mu =1.5\right)$ lens in air is 20 cm. If it is dipped in water $\left(\mu =\frac{4}{3}\right)$, its focal length in water will be:

A plane-convex lens fits exactly into a plano-concave lens. Their plane surfaces are parallel to each other. If lenses are made of different materials of refractive indices μ₁ and μ₂ and R is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is:
1. R/2(μ_{1 +} μ₂)
2. R/2(μ_{1 -} μ₂)
3. R/(μ_{1 -} μ₂)
4. 2R/(μ₂ - μ₁)