A person can see clearly objects only when they lie between 50 cm and 400 cm from his eyes. In order to increase the maximum distance of distinct vision to infinity, the type and power of the correcting lens, the person has to use, will be

  • Option 1)

    convex, + 2.25 diopter

  • Option 2)

    concave, - 0.25 diopter

  • Option 3)

    concave, - 0.2 diopter

  • Option 4)

    convex, + 0.15 diopter

 

Answers (1)

As we learnt in

Optical power of a mirror -

P= -frac{1}{f}

 

- wherein

f= focal length with sign and is in meters unit of power = diopter

 

 

 

This person wants to increase the farther distance from 400 cm to \infty. Hence he must use a lense who will form image of an object which is place at infinity to 400 cm.

Hence focal length of the lens = 400 cm.

It must be concave 

P = \frac{-1}{400}cm = \frac{-1}{4} D

P=-0.25 \:\: D

 


Option 1)

convex, + 2.25 diopter

This solution is incorrect.

Option 2)

concave, - 0.25 diopter

This solution is correct.

Option 3)

concave, - 0.2 diopter

This solution is incorrect.

Option 4)

convex, + 0.15 diopter

This solution is incorrect.

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