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Why does a concave mirror used as a torch reflector produce a parallel beam?
AThe mirror has the same radius as the bulb (option)
BThe bulb is at infinity
CThe bulb is at the centre of curvature
DThe bulb is placed at the focus, so rays emerge parallel after reflection
Answer & Solution
Correct answer: D. The bulb is placed at the focus, so rays emerge parallel after reflection
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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