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.

Answer 1

(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.

$P=\frac{1}{f}$

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.

$1D=\frac{1}{1m}$

c) Here, $f_1=20cm$, $f_2=40cm$ and $f_3=-50cm$

So, $\frac{1}{f_{eq}}=\frac{1}{f_1}+\frac{1}{f_2}+\frac{1}{f_3}=\frac{1}{20}+\frac{1}{40}+\frac{1}{-50}=\frac{11}{200}$

$\Rightarrow f_{eq}=\frac{200}{11}=18.18cm$

$P_{eq}=\frac{1}{f_{eq}inmetres}=\frac{1}{0.18}=5.55D$