Question

you can use a magnifying glass to make the light from the sun converge at a point. why does a piece of paper placed at the point start burning after a while?

Answer 1

The magnifying glass focus the sun's light to fall on a certain place by converging with the use if convex lens

Answer 2

The paper start burning because the magnifying glass converges the sunlight at a specific point. The temperature at that point increases and the paper catches fire.