Question

Under what condition does a biconvex lens of glass having a certain refractive index acts as a plane glass sheet when immersed in a liquid

Answer 1

When the refractive index of a lens is equal to the refractive index of a liquid in which it is immersed,then the lens acts as a plane glass sheet. When the refractive index are same,then the focal length of a lens becomes infinity and hence it acts as a plane glass sheet.