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

What is convex lens ?

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

When an object moves towards a convex lens, the size of the image decreases.

How can geothermal energy be used to generate electricity?

"In between a fixed object and a fixed screen, a convex lens can cast two images at two different positions of the lens". Taking this to be a true statement show that product of the image sizes is equal to the square of the object size.