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Why does a concave mirror used as a torch reflector produce a parallel beam?
Answer & Solution
Correct answer: D.
1. The defining property of the principal focus is that incoming parallel rays converge there.
2. By reversibility, rays diverging from the focus emerge parallel to the principal axis.
3. Placing the torch bulb at the focus thus produces a parallel beam — exactly what a torch wants.
_Source: NCERT Class 10 Science, Ch 9 "Light Reflection and Refraction", §9.2.2_
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