Question

# (a) What do you understand by the power of a lens? Name one factor on which the power of a lens depends. (b) What is the unit of power of a lens? Define the unit of power of a lens. (c) A combination of lenses for a camera contains two converging lenses of focal lengths 20 cm and 40 cm and a diverging lens of focal length 50 cm. Find the power and focal length of the combination.

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Solution

## (a) Th ability of a lens to converge/diverge a beam of light rays is known as its power. It is defined as the reciprocal of the focal length. P=1f Power depends on focal length. b) The SI unit of power of a lens is Dioptre. It is denoted by the letter "D". 1 dioptre is the power of a lens of focal length 1 m. 1 D=11 m c) Here, f1=20 cm, f2=40 cm and f3=−50 cm So, 1feq=1f1+1f2+1f3=120+140+1−50=11200 ⇒feq=20011=18.18 cm Peq=1feq in metres=10.18=5.55 D

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