Question

power of the lens is - 4D then find its focal length is ​

Answer 1

Given ,

The power of the lens is -4 D

We know that ,

The reciprocal of focal length is called power of lens

It is denoted by " P "

$\boxed{ \sf{Power \: of \: lens \: (P) = \frac{1}{focal \: length \: (in \: m) } }}$

The SI unit of power of lens is " dipotre "

Thus ,

-4 = 1/f

f = -1/4

f = -0.25 m

Therefore ,

The focal length of lens is 0.25 m

Answer 2

AnswEr:-

Your Answer Is 25 cm.

ExplanaTion:-

Given:-

• Power of a lens = -4 D.

To Find:-

• The focal length of the lens.

Formula Used:-

$\tt\longrightarrow p = \dfrac{1}{f}.$

$\bf\longrightarrow f = \dfrac{1}{p} .$

Where,

• p = Power of the lens (in Dioptre).

• f = Focal length (in metres).

So Here,

• p = -4 D.

• f = ?? (To Find).

Now,

By putting the above values in formula we get:-

$\tt\mapsto f = \dfrac{1}{p} .$

$\tt\mapsto f = - \dfrac{1}{4} \: metre.$

$\tt\mapsto f = - 0.25 \: metre.$

$\tt\mapsto f = - (0.25 \times 100) \: cm.$

$\bf\mapsto f = - 25 \: cm.$

Therefore the focal length of the lens is 25 cm.

(Also as the lens's Focal Length is negative so it is a concave lens).