A concave spherical refractive surface with radius R separates a medium of refractive index `5//2` from air. As an object is approaching the surface from far away from the surface along the central axis, its image

Options

A. always remains real
B. always remains virtual
C. changes from real to virtual at a distance `2R//3` from the surface
D. changes from virtual to real at a distance `2R//3` from the surface

Answer

b.

Explanation

`(2.5)/(v)-(1)/(-u)=(1.5)/(-R)`
`v=-(2.5uR)/(1.5u+R)`
`vlt0` for all values of u, so the image remains virtual.