A student suffering from myopia is not able to see distinctly the objects placed beyond 5 m.

Question

(i) A student suffering from myopia is not able to see distinctly the objects placed beyond 5 m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams, explain 

(a) why the student is unable to see distinctly the objects placed beyond 5 m from his eyes. 

(b) the type of the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens. 

(ii) If, in this case, the numerical value of the focal length of the corrective lens is 5 m, find the power of the lens as per the new Cartesian sign convention.

Answer 1

(i) The defect may have arisen due to following reasons: 

•  excessive curvature of the eye lens 
•  elongation of the eyeball 

(a) Because the image of the object is formed in front of the retina and not on it.

(b) Concave or diverging lens is used to restore proper vision. It will bring the image back on the retina.

(ii) f = – 5 m (since lens is concave) 

In case of myopia, 

P = 1 / f(metre),= – 1 / 5 

P = –0.2 D