Question

(a) What is meant by ‘power of a lens’?; (b) State and define the S.I. unit of power of a lens.; (c) A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close contact with each other. Calculate the lens power of this combination.(All India - 2011)

Answer 1

Explanation:

• power of a lens is defined as the reciprocal of focal length in metre . - s. I unit is Dioptre.

Answer 2

Power of lens

(a) Meaning:

• The reciprocal of the focal length of the lens is called its power of lens.

(b) S.I unit:

• Its S.I unit is dioptre and it is denoted by symbol 'D'.
• As the value of 'D' denotes the power of the lens, the value of 'f' denotes the focal length.
• It is mathematically represented as 1 D = $1 m^{-1}$.

(c) Power of the lens calculation:

Given:

Focal length of convex lens = 25 cm = 0.25 m

Focal length of concave lens = 10 cm = 0.1 m

Calculation:

To find the power of a lens with the given combination, the power of both the lens must be calculated individually.

∴ Power of convex lens = $\frac{1}{f} = \frac{1}{0.25}= 4$

Power of concave lens = $\frac{1}{-f} = \frac{1}{(-0.1)} = -10$

Power of lens with combination = 4 + (-10) = 4 -10 = -6 D

Conclusion:

The lens power of given combination is calculated as -6D.

Learn more about such concept

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