Question

Power of the lens is -40, its focal length is
         a. 4m
         b. -40m
         c. -0.25m
         d. -25m​

Answer 1

Answer:

Power of the lens is -40, its focal length is

         a. 4m

Answer 2

Given :

Power of the lens = -40 D.

To Find :

The focal length of the lens

Solution :

We know that,

The power of a lens is inversely proportional to its focal length so, a lens of short focal length has more power whereas a lens of long length has less power.

The unit of power of a lens is dioptre.

» The power is the reciprocal of the focal length that is

$\boxed{ \bf P = \dfrac{1}{f} }$

where,

• P denotes power of the lens
• f denotes the focal length

By, substituting the given values in the formula,

$\dashrightarrow\sf P = \dfrac{1}{f}$

$\dashrightarrow\sf - 40 = \dfrac{1}{f}$

$\dashrightarrow\sf f = \dfrac{1}{ - 40}$

$\dashrightarrow\sf f = - 0.025 \: cm$

Thus, the focal length of the lens is - 0.025cm.

The concave lens has a negative focal length so, the power of a concave lens is negative.