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Question
Draw a ray diagram to show how a converging lens can form an image of the sun. Hence give a reason for the term ‘burning glass’ for a converging lens used in this manner.
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Solution
The sun is at infinity, so the convex lens forms its image at the 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 the sufficient heat of the sun's rays, the paper burns. Hence, this lens is termed 'burning glass'.
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