Question

A glass lens of refractive index 1.5 is placed in a trough of liquid. What must be the refractive index of the liquid in order to mark the lens disappear?

Answer 1

Liquid should have same refractive index as of glass i.e., 1.5 so the light passes from liquid will not deviate when passing through lense .

And hence lense will not appear from any corner of liquid. Thus

Refractive index = 1.5