A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
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- For distance vision, given f = ?, P = -5.5 D
$\text{P}=\frac{1}{\text{f}} \ \text{or} \ \text{f}=\frac{1}{\text{P}}$
or $\text{f}=\frac{100}{-5.5} =-18.2 \ \text{cm}$
- For near vision, given f = ?
Using the relation
$\text{P}=\frac{1}{\text{f}} \ \text{or} \ \text{f}=\frac{1}{\text{P}}$
or $\text{f}=\frac{100}{1.5} =66.7 \ \text{cm}$
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