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A small bulb is placed exactly at the principal focus of a convex lens. What kind of beam emerges from the other side of the lens?
AA parallel beam of light
BA convergent beam of light
CA divergent beam of light
DA coloured beam of light
Answer & Solution
Correct answer: A. A parallel beam of light
1. By the lens rule, a ray passing through the principal focus emerges parallel to the principal axis.
2. A source placed at the focus sends rays that all leave parallel to the axis.
3. So a parallel beam emerges from the lens.
4. Convergence would require the source to be beyond the focus, so that option is wrong.
_Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 2 Optics "Refraction through a Convex Lens", p.27_
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