Question

18. A person suffering from defective vision can see objects clearly only beyond 100 cm from the eye.
Calculate the power of the lens in diopter required so that he can see clearly the object placed at least
distance of distinct vision (D = 25 cm).​

Answer 1

The power of the lens required is 3 Diopter

Explanation:

Given:

A person suffering from defective vision can see objects clearly only beyond 100 cm

To find out:

The power of lens required to see the object placed at 25 cm

Solution:

In order to see the objects placed at 25 cm clearly, the image of the object should be formed at least at 100 cm

Therefore,

Using the lens formula

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

Here,

v = -100 cm

u = -25 cm

Therefore,

$\frac{1}{-100}-\frac{1}{-25}=\frac{1}{f}$

$\implies \frac{1}{f}=\frac{4-1}{100}$

$\implies f=\frac{100}{3}$ cm

or $f=\frac{1}{3}$ m

Therefore, the power of the lens

= 1/Focal length of the lens in m

$=\frac{1}{1/3}$

$=3$D

Hope this answer is helpful.

Know More:

Q: A person cannot see objects clearly when they are placed at a distance less than 30 cm from the what should be the power of corrective lens and what type of lens is used to correct the defect ?

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Q: A person cannot see the object distinctly when placed at a distance less than 50cm

a) identify the defect of vision

b) calculate the power and nature of the lens he should be using to see clearly the object placed at a distance of 25cm from his eyes.

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