The focal length of a lens depends upon :

1. the refractive index of the material of lens relative to its surrounding medium
2. the amount of light entering the lens
3. the radius of curvature of the two surfaces of lens
4. both (1) and (3)

If a lens is placed in water instead of air, its focal length :

1. increases
2. decreases
3. remains the same
4. can both increase or decrease

The focal length of a thin convex lens is …………… than that of a thick lens.

1. less
2. more
3. same
4. none of the above

The radius of the sphere whose part is the lens surface is called the …………… of that surface.

1. centre of curvature
2. focal length
3. pole
4. radius of curvature