Under what conditions will a convex lens have a negative and a concave lens a positive power?

Under what conditions a concave lens can behave as a converging lens?

Which lens has positive and which lens has negative power?

An optical (eye specialist)prescibes spetacles is a pateint with a combination of a convex lens of focal length 40 cm nd concave lens 25 cm.The power of spectacles is:

under what condition does a biconvex lens of glass having a certain refractive index act as a plane glass sheet,when immersed in a liquid?

How can a convex lens behavae like a diverging lens?