You are provided with two lenses of focal length 20 cm and 30 cm, respectively. Which lens will you use to obtain more convergent light?
We know that Power of lens is defined as the efficiency with which a lens can converge or diverge the light ray.
Also P (in diopter) = 
Hence power is inversely proportional to the focal length
Therefore the lens with focal length 20cm will have more power than the lens with focal length 30cm. Therefore the lens with focal length 20cm will give more convergent light.
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