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: