A person is unable to see objects distinctly placed within 50 cm from his eyes.
(a) Name the defect of vision the person is suffering from and list its two possible causes.
(b) Draw ray diagram to show the defect in the above case.
(c) Mention the type of lens used by him for the correction of the defect and calculate its power. Assume that the near point for the normal eye is 25 cm.
(d) Draw a labelled diagram for the correction of the defect in the above case.

Explanation

(a) Hypermetropia is the defect of vision. This defect arises either because-
(i) The focal length of the eye lens is too long.
(ii) (ii) Decrease in the size of the eyeball.
(b)
(c) Convex lens can be used to correct this defect.
Power `(P) = (1)/(f)` object distance ` u = - 25 cm `:
Image distance , = - 50 Focal lenght = f
Lenes formula , `(1)/(f) = (1)/(v)- (1)/(u)`.
`= (1)/(-50) - (1)/(-25) = (-1-(-2))/(50) = (-1+2)/(50) = (1)/(50) = 50 cm or 0.5m`
` therefore f = (1)/(f) = (1)/(0.5) = (10)/(5) = 2D`