Question

A person uses lenses of power −0.5 D in his spectacles for the correction of his vision.

1. Name the defect of vision the person is suffering from.
2. List two causes of this defect.
3. Determine the focal length of the lenses used in the spectacles.

Answer 1

a. The person is suffering from myopia (near sightedness).

b. Two causes of myopia are:

1. Elongation of the eyeball: The image forms in front of the retina instead of on it.
2. Increased curvature of the eye lens: The lens becomes too curved, leading to excessive convergence of light.

c. Given: P = − 0.5 D

The focal length (f) of a lens is given by the formula:

P = `1/f`

Where P is the power of the lens in diopters (D) and f is the focal length in meters.

f = `1/P`

f = `1/(−0.5)`

= −2 meters

Thus, the focal length of the lenses used in the spectacles is −2 meters (or −200 cm). The negative sign indicates a concave lens, which is used to correct myopia.