A person needs a lens of power –2.5 dioptres for correcting his vision. Name the defect of vision he is…
A person needs a lens of power –2.5 dioptres for correcting his vision. Name the defect of vision he is suffering from. Which lens will he be using for the correction? Also, find the focal length of lens.
Solution
Given $P = -2.5\ \text{D}$ (negative power).
A negative power means a **concave (diverging) lens**, so the person is suffering from **myopia (near-sightedness)**.
Focal length:
$f = \dfrac{1}{P} = \dfrac{1}{-2.5} = -0.4\ \text{m} = -40\ \text{cm}$
So he uses a concave lens of focal length $40\ \text{cm}$.