Question

What us meant by power of a lens? Write its SI unit. A student uses a lens of focal length 40cm and another of -20cm.Write the nature and power of each lens.

Answer 1

Ability of eye lens to increase or decrease it's focal length called power of lens.
SI Unit - Diopter (D)
(i) F = 40/100m
= 0.4m
Nature = Real and Inverted OR Converging lens
Power = 1/F
= 1/0.4
= 2.5D

(ii) F = -20/100m
= -0.2m
Nature = Virtual and Erect OR Diverging lens
Power = 1/F
= 1/-0.2
= -5D

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