Question

(a) The far point of a myopic person is `80 cm`. In front of the eye. What is the power of the lens required to enable him to see very distant objects clearly ?
(b) In what way does the corrective lens help the person above ? Does the lens magnify very distant objects ? Explain carefully.
( c) The person above prefers to remove his spectacles while reading a book. Explain why ?

Answer 1

(a) To see very distant objects clearly, the person needs a conave lens of focal length
`f = - 80 cm`
`P = (100)/(f) = (100)/(-80) = - 1.25 D`
(b) No, the corrective lens is concave and infact it reduces the size of the image. The eye is able to see distant objects not because the corrective lens magnifies the object, but becasue it brings the object at the far point `F` of teh eye (forming virtual image at `F` ) which can then be focussed on the retina by the eye lens.
( c) The myopic person may have a normal near point `(~~ 25 cm)`. To read a book with his spectacles (for distant vision), he has to hold the book at a distance more than `25 cm` so that concave lens produces the image not closer than `25 cm`.
As angular size of object at a distance `gt 25 cm` is les than the angular size at `25 cm`, the person prefers to remove his spectacles while reading.