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?

Answer 1

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 $=\frac{1}{P}$

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 $=\frac{1}{-5.5}=-0.181m$

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 $=\frac{1}{P}$

$=\frac{1}{+1.5D}=+0.667m$

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