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Question

A person needs a lens of power -5.5dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?


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Solution

Step 1: Given data

Power of the lens used for correcting distant vision =-5.5D

Power of the lens used for correcting near vision =+1.5D

Step 2: Formula used

The focal length of the lens =1P

Step 3: Focal length of the lens required for correcting distant vision

Power of the lens used for correcting distant vision =-5.5D

The focal length of the lens =1-5.5=-0.181m

Step 4: Focal length of the lens required for correcting near vision

Power of the lens used for correcting near vision =+1.5D

The focal length of the lens =1P

=1+1.5D=+0.667m

Hence, the focal length of lens for correcting distant and near vision are -0.181mand+0.667m respectively.


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