A convex lens is said to have a real focus and a concave lens a virtual focus. Explain.

What is concave lens ?

What is convex lens ?

Can a concave lens produce a real image?

Define the principal focus of a concave mirror.

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.

Find the power of a concave lens of focal length 2 m ?