What is meant by power of a lens? Write its SI unit. A student uses a lens of focal length 40cm and another of -20cm. Write the nature and power of each lens.
CBSE DELHI - OUTSIDE DELHI - FOREIGN SET 1 2018
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Power of lens:The power of a lens is the measure of degree of convergence or divergence of light rays falling on it. The SI unit of power of a lens is dioptre denoted by ‘D’.
The power of a lens is given by,
$\text{P}=\frac{1}{\text{f (in metres)}}$
$\therefore$ For lens of focal length 40cm, f = 0.4m
$\therefore\ \text{P}=\frac{1}{0.4}=+2.5\text{D}$
The lens is convex lens and is used to correct hypermetropia.
For lens of focal length -20cm, f = 0.2m
$\therefore\ \text{P}=\frac{1}{-0.2}=-5.0\text{D}$
The lens is concave lens and is used to correct myopia.
The power of a lens is given by,
$\text{P}=\frac{1}{\text{f (in metres)}}$
$\therefore$ For lens of focal length 40cm, f = 0.4m
$\therefore\ \text{P}=\frac{1}{0.4}=+2.5\text{D}$
The lens is convex lens and is used to correct hypermetropia.
For lens of focal length -20cm, f = 0.2m
$\therefore\ \text{P}=\frac{1}{-0.2}=-5.0\text{D}$
The lens is concave lens and is used to correct myopia.
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