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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 for correcting distant vision, P= -5.5D

Power correcting near vision P1= +1.5D

Step 2. Formula used,

P=1f

P is the power of lens that is how well a lens can converge or diverge light

f is the focal length of the lens used

Step. 3 Calculating the focal length for distant vision,

P=1f

f=1P

f=1-5.5=-0.182m

Step 4. Calculating focal length for near vision,

f=1P1

f=11.5=0.66m

We can see that power and focal length are inversely proportional that is if the power is more then the focal length will be lesser

Hence Focal length for distant vision= 0.185m and Focal length for near vision= 0.66m


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