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

Explanation

Solving we find that the person should use a concave lens of focal length= -80 cm, i.e., of power = - 1.25 dioptres.
b. No. The concave lens, in fact, reduces the size of the object, but the angle subtended by the distant object at the eye is the same as the angle subtended by the image (at the far point ) at the eye. THe eye is able to see distant objects not because the corrective lens magnifies the object, but because it brings the object (i.e., it produces virtual image of the object ) at the far point of the eye which then can be focussed by the eye-lens on the retina.
c. The myopic person may have a normal near point, i.e, about 25 cm (or even less ) . In order to read a book with the spectacles , such a person must keep the book at a distance not closer than 25 cm. The angular size of the book (or its image ) at the greater distance is evidently less than the angular size when the book is placed at 25 cm and so spectacles are needed Hence, the person prefers to remove the spectacles while reading .