The paper burns because the parallel rays of sunlight are converged by the lens to form a sharp and bright spot on the paper. This spot acts as a real image of the Sun, concentrating sunlight at that point and generating heat, eventually causing the paper to catch fire.
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When sunlight is focused through a lens onto a piece of paper, it can lead to burning due to the concentration of solar energy. The lens converges sunlight, concentrating it onto a small spot on the paper. This intensified light increases the temperature of the focused area, reaching levels sufficient to ignite the paper. The process involves the absorption of sunlight by the paper, converting it into heat energy. As the temperature rises, the paper undergoes combustion, releasing gases and undergoing a chemical reaction with oxygen, resulting in burning. This phenomenon demonstrates the power of concentrated sunlight to generate intense heat.