Question

A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?​

Answer 1

$\bf{\underline{Given}}:-$

• Lens of power (p) = 1.5 D

$\bf{\underline{To\:Find}}:-$

• The focal length of the lens.

$\bf{\underline{Solution}}:-$

we know that,

$\blue{\bigstar}\:\:{\underline{\boxed{\bf\red{Power\:(p)=\dfrac{1}{f\:(in\:metres)}}}}}$

$\rm\longrightarrow\:1.5=\dfrac{1}{f}$

$\rm\longrightarrow\:f=\dfrac{1}{1.5}\:m$

$\rm\longrightarrow\:f=\dfrac{1}{1.5}\times\:10\:m$

$\rm\longrightarrow\:f=\cancel\dfrac{10}{15}\:m$

$\rm\longrightarrow\:f=\dfrac{2}{3}\:m$

$\rm\longrightarrow\:f=0.67\:m$

Hence,the focal length of the lens will be 0.67 m.

The power of this lens is positive with a plus sign therefore,this is a convex lens or converging lens.

$\blue{\bigstar}\:\:{\underline{\boxed{\bf\pink{More\: Information}}}}$

➝ Power of lens :- The power of a lens is defined as the reciprocal of its focal length in metres.

• A lens of short focal length has more power whereas a lens of long focal length has less power.

• The unit of power of lens is dioptres.

• A power of convex lens is positive.

Answer 2

Answer:

$The \: power \: of \: this \: lens \: is \: positive , \\ therfore, \: this \: is \: a \: convex \: lens \: or \: \\ converging \: lens. \: now, \\ Power, p = \frac{1}{f(in \: metrs)} \\ so \: \: \: \: \: + 1.5 = \frac{1}{f} \\ or \: \: \: f = \frac{1}{ + 1.5} \: m \\ f = + \frac{1}{1.5} \times 100cm \\ \\ So, \: \: \: focal \: length \: of \: lens \: f = + \: 66.7cm$