First slide

The optical instruments

Question

A man can see the objects upto a distance of one metre from his eyes. For correcting his eye sight so that he can see an object at infinity, he requires a lens whose power is
or
A man can see upto 100 cm of the distant object. The power of the lens required to see far objects will be

Moderate

A

+ 0.5 D
B

+ 1.0 D
C

+ 2.0 D
D

– 1.0 D

Solution

The image of object at infinity should be formed at 100 cm from the eye
$\frac{1}{f} = \frac{1}{- 100} - \frac{1}{- \infty} = - \frac{1}{100} So the power = \frac{- 100}{100} = - 1 D (Distance is given in cm but P = \frac{1}{f} in metres)$

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