Question

A lens is thicker in the middle than at the edges. Can it be used to burn a piece of paper by using sunlight?

Answer 1

A lens which is thicker in the middle than at the edges is a converging or convex lens. It converges all the light rays coming from a distant object to its focus. As the sun is a distant object for a convex lens, so the lens will converge all the light rays coming from the sun and will focus them on the focal point of the lens. Such a lens can be used to burn a piece of paper because sunlight focussed on a single point can produce heat and can even start a fire.