Under what condition can a convex lens form a virtual image?

Can a concave lens produce a real image?

What is convex lens ?

When an object is placed at a distance 6 cm from a concave lens a virtual image is formed at a distance 4cm from it. Find the focal length of the lens.

what is reversible process ?under what condition can a process be reversible ?

How can a convex lens be used to light up a fire?