Question

Draw a ray diagram to show how a converging lens can form an image of the sum. Hence give a reason for the term 'burning glass' for a converging lens used in this manner.

Answer 1

The sun is at infinity so convex lens forms its image at second focal point which is real and very much diminished in size.

While using the convex lens as burning glass, the rays of light from the sun (at infinity) are brought to focus on a piece of paper kept at the second focal plane of the lens. Due to sufficient heat of the sun rays, the paper burns. Hence this lens is termed as ‘burning glass’.