Question

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?​

Answer 1

Given:-

• 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.

To Find:-

• Distant Vision.
• Near Vision.

Solution:-

1)

A person needs a lens of power –5.5 dioptres for correcting his distant vision.

$\sf \: P = - 5.5D$

We know that

$P = \frac{1}{f}$

$= > f = \frac{1}{P}$

$= > f = \frac{1}{ - 5.5}$

$= > f = - 0.18m$

∴ The focal length of the lens for correcting distant vision is – 0.18 m.

2)

For correcting his near vision he needs a lens of power +1.5 dioptre.

$\sf \: P = + 1.5D$

Then,

$= > f = \frac{1}{P}$

$= > f = \frac{1}{1.5}$

$= > f = 0.66m$

∴The focal length of the lens for correcting near vision is 0.66 m