The paper catches fire due to the concentration of sunlight at a sharp, bright spot on the paper, which is the image of the Sun formed by the concave mirror. This point is the focus of the mirror, and the intense heat generated at this concentrated spot ignites the paper.
Why does the paper eventually catch fire when exposed to sunlight through a concave mirror?
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When sunlight is focused onto a small area using a concave mirror, the concentrated solar energy results in an intense heat spot. This heat spot can raise the temperature significantly, causing the paper to combust and catch fire. The concave mirror converges sunlight to a focal point, concentrating the energy in a smaller region, amplifying its heating effect. This process is akin to using a magnifying glass to focus sunlight onto a specific point. The high temperature at the focal point exceeds the ignition temperature of the paper, leading to combustion and the eventual ignition of the paper.