Question

A man uses a concave lens as a spectacles whose focal length is 20 cm. What is the power of lens in dioptre? (D.B.-16)

• A. -5
• B. -0.5
• C. +0.5
• D. +5

Answer 1

Answer: (A)

-5

Answer 2

The power of a lens is defined as the reciprocal of its focal length in meters. The formula is given by:

$P = \frac{1}{f}$

where $P$ is the power in Dioptres (D) and $f$ is the focal length in meters.

The given lens is a concave lens, and its focal length is 20 cm. For a concave lens, the focal length is considered to be negative. So, the focal length $f = -20$ cm.

To use the formula for power, we need to convert the focal length from centimeters to meters:

$1 \text{ meter} = 100 \text{ centimeters}$

So, $f = -20 \text{ cm} = \frac{-20}{100} \text{ meters} = -0.20 \text{ meters}$.

Now, we can calculate the power of the lens using the formula $P = \frac{1}{f}$:

$P = \frac{1}{-0.20 \text{ m}}$ $P = \frac{1}{-1/5 \text{ m}}$ $P = -5 \text{ m}^{-1}$

The unit $\text{m}^{-1}$ is called Dioptre (D).

So, $P = -5$ D.

The power of the concave lens is -5 Dioptres.