Question

Doctor prescribes lens of power -0.5 D. Then the type of lens and it's focal length is ____.

convex lens , -2m

concave lens, -2m

convex lens, 2m

concave lens,2m​

Answer 1

Given :

The power of lens : -0.5 D.

To find :

The focal length.

Solution :

The doctor prescribes concave lens,

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,

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

$\dashrightarrow\sf - 0.5 = \dfrac{1}{f}$

$\dashrightarrow\sf f = \dfrac{1}{ - 0.5}$

$\dashrightarrow\sf f = - 2 \: m$

Thus, the focal length is - 2 m.