Define the power of a lens. Also mention its SI unit. A student uses a lens of focal length 30 cm and another of −10 cm. Write the nature and power of each lens.

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Detailed Solution

The power of the lens is defined as the reciprocal of the focal length of the lens expressed in metres.

$P = 1 / f o c a l l e n g t h$

The SI unit of power is Dioptre denoted by D. Focal length of the first lens,

$F_{1} = 30 c m$

$= 0.3 m$

Since, the focal length is positive, it is a convex lens.

Power of lens,

$P_{1} = 1 / F_{1}$

$= 1 / 0.3$

$= 3.33 D$

Focal length of the second lens,

$F_{2} = - 10 c m$

$= - 0.1 m$

Since the focal length is negative, it is a concave lens.

Power of lens,

$P_{2} = 1 / F_{2}$

$= 1 / (- 0.1)$

$= - 10 D$

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