(a) State two main causes of a person developing near sightedness. With the help of a ray diagram, suggest…
(a) State two main causes of a person developing near sightedness. With the help of a ray diagram, suggest how he can be helped to overcome this disability?
(b) The far point of a myopic person is 150 cm in front of the eye. Calculate the focal length and power of a lens required to enable him to see distant objects clearly.
Solution
a. Elongation of the eye ball, and excessive curvature of the cornea. He should be advised to use a **concave lens** of suitable focal length. In the ray diagram, the image of a distant object forms in front of the retina for the defective eye; the concave lens diverges the rays so that the image falls on the retina.
b. For a myopic eye the corrective lens must bring the image of a distant object to the far point, so $f = -150\ \text{cm}$.
$P = \dfrac{100}{f\ (\text{cm})} = \dfrac{100}{-150} = -0.66\ \text{D}$
A concave lens of focal length $150\ \text{cm}$ and power $-0.66\ \text{D}$ is required.
