Question

When and how does a convex lens behave like a concave lens?

Answer 1

When convex lens is immersed in a liquid whose refractive index is greater than that of the lens material. the radii of curvatures in both the lens remain same and convex ens behave like concave lens


From the law of refraction, incident ray, refracted ray and normal at surface are lies in one plane.

Now

when light ray enters from rarer to denser medium, it turn towards the normal, and from dencer to rarer it turns away to the normal.
and it is depends upon the refractive index of medium (u).

This reflective index of medium is a relative value which is given by snells law
(u)= sin(i)/sin(r). where (i)&(r) is the angle of incident ray and reflected ray. So denser medium have greater value of (u).

That's why the rarer medium convex lens placed in denser medium behaves like as concave lens and Concave lens behaves like Convex lens.

Because for those medium the relative value of refractive index is changed according to the situation.

Hope it Helps..

Cheers & Peace :)