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Question
Define power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of –50 cm. What is the nature of the lens and its power used by each of them?
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Solution
Power of a lens indicates the converging effect of the lens. It is the reciprocal of the focal length of the lens.
Mathematically, `"Power" ("P") = 1/"f"`
The unit of power is dioptre (D).
The nature and power of the lens in each case are:
When the focal length of the lens is 50 cm,
`("P") = 1/"f"`
`= 1/0.5`
= 2D
This lens has positive value of focal length. So, it is convex lens.
When the focal length of the lens is -50 cm,
`("P") = 1/"f"`
`= 1/(-0.5)`
= -2 D
This lens has negative value of focal length. So, it is concave lens.
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