Question

A man with defective eyes cannot distinctly see an object at a distance more than $60cm$ from his eyes . The power of the lens to be used will be

• A. $60D$
• B. $-60D$
• C. $-1.66D$
• D. $\frac{1}{1.66}D$

Answer 1

The correct option is C

$-1.66D$

Step 1: Given:

Distance of a maximum visible point $=f=60cm$ $=0.6m$

Step 2: Formula used

$P=-\frac{I}{f}D$

$f=$ focal length in m

$D$ is unit of power in dioptre

Step 3: Calculation

The power of the lens in the given case is,

$P=-\frac{1}{0.6}$

As the person is unable to see the distant object, thus, a concave lens is used in the given case of myopia.

The negative sign is indicating the concave lens in the value of power.

Hence, option C is the correct answer i.e. $-1.66D$.