Question

The power of spectacles lens required for a person
is -5 dioptre when separation between spectacles
and eye is 2 cm. What will be the power of contact
lens required by him?
(1) -5.25 D
(2) -4.54 D
(3) -3.2 D
(4) -5.75 D​

Answer 1

Answer:

Power of the lens required is -4.54 D

Explanation:

Power of the lens required is -5D

so we have

$\frac{1}{f} = 5$

$f = 0.2 m = 20 cm$

so the distance upto which it can see clearly is given as

$\frac{1}{d_i} + \frac{1}{d_o} = \frac{1}{f}$

now we know that

$\frac{1}{d_i} + \frac{1}{infinite} = \frac{1}{20}$

$d_i = 20 cm$

now when he use contact lens then we have the distance given as

$d_i = 20 + 2 = 22 cm$

now we have

$\frac{1}{22} + \frac{1}{infinite} = \frac{1}{f}$

$f = 22 cm$

$P = -4.54 D$

#Learn

Topic : Eye defect

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