The focal length of a glass lens in air is 20 cm. If it is dipped in water , its focal length in water will be:
1. | 80 cm | 2. | 40 cm |
3. | 60 cm | 4. | 20 cm |
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(μ1 + μ2)
2. R/2(μ1 - μ2)
3. R/(μ1 - μ2)
4. 2R/(μ2 - μ1)