(a) What is short-sightedness? State the two causes of short-sightedness (or myopia). With the help of ray diagrams, show:
1. the eye-defect short-sightedness.
2. correction of short-sightedness by using a lens.
(b) A person having short-sight cannot see objects clearly beyond a distance of 1.5 m. What would be the nature and power of the corrective lens to restore proper vision?
(a) Short Sightedness is that defect of vision due to which a person can see nearby objects clearly but cannot see distant objects clearly.
Causes:
1. Excessive curvature of the eye lens
2. Elongation of the eyeball
Ray diagram for:
(i) eye-defect short sightedness
(b) Concave lens should be used to restore proper vision.
Calculation of power of the lens:
Here, far point of myopic = 1.5 m
The object kept at infinity can be seen clearly if the image of this object is formed at 1.5 m.
So, object distance, u = ∞
Image distance, v = 1.5 m
\(\frac{1}{v}\) - \(\frac{1}{u}\) = \(\frac{1}{f}\)
\(\frac{1}{-1.5}\) - \(\frac{1}{ ∞}\) = \(\frac{1}{f}\)
f = 1.5 m
P = \(\frac{1}{f}\) = \(\frac{1}{-1.5}\)
= -0.67 D