Question

what is meant by power. a lens? write uts SI unit. a student uses a lens of focal length 40 cm and another of -20 cm. write the nature

Answer 1

$\mathfrak{\large{\underline{\underline{Answer:-}}}}$

The degree of convergence or divergence of light rays by the lens is called it's power.

or,

The reciprocal of focal length is known as power.

It's Si unit is Dioptre.

By using formula of power,

★ $\boxed{\sf{ </strong><strong>P</strong><strong> </strong><strong>=</strong><strong> \frac{1}{f} (100)</strong><strong> </strong><strong>}}$

where, it is multiplied by 100 to convert in cm.

Now focal length = 40 cm

$\implies$ $\bold{</strong><strong>P</strong><strong> </strong><strong>=</strong><strong>\frac{100}{40} </strong><strong> </strong><strong> }$

$\implies$ $\bold{</strong><strong>P</strong><strong> </strong><strong>=</strong><strong> </strong><strong>2</strong><strong>.</strong><strong>5</strong><strong> </strong><strong>D</strong><strong>}$

Nature:- It is convergence lens because it's power is positive or convex lens.

Now focal length = -20 cm

$\implies$ $\bold{</strong><strong>P</strong><strong> </strong><strong>=</strong><strong> </strong><strong>\frac{ - 100}{20} </strong><strong>}$

$\implies$ $\bold{</strong><strong>P</strong><strong> </strong><strong>=</strong><strong> </strong><strong>-</strong><strong>5</strong><strong> </strong><strong>D</strong><strong>}$

Nature :- It is a divergence or concave lens because it's power is in negative.