What is meant by the power of a lens ? What is its S.I. unit ? Name the type of lens whose power is positive.
The image of an object formed by a lens is real, inverted and of the same size as the object. If the image is at a distance of 40 cm from the lens, what is the nature and power of the lens ? Draw ray diagram to justify your answer.
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Answer:
Power of a lens is defined as its ability to converge or diverge light rays.
SI unit: Dioptre, denoted by D
Nature of lens: Convex
Power of lens: 0.05 D
Refer to the attachment for the diagram.
Explanation:
Given:
- m = -1
- u -40 cm
- v = + 40
To find:
- focal length, f = ?
- power, P = ?
Solution:
Since, numerically, v = u = 40 cm,
The object and the image both are at 2F.
So, F = 40/2
F = + 20 cm
Power of lens, P = 1/f = 1/20 = 5/100 = + 0.05 D
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