A plane convex lens (µ = 1.5) has a radius of curvature \(10~\mathrm{cm}\). It is silvered on its plane surface. The focal length of the lens after silvering is:
1. 10 cm
2. 20 cm
3. 15 cm
4. 25 cm
A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length will:
1. | become zero. |
2. | become infinite. |
3. | become small, but non-zero. |
4. | remain unchanged. |