Question

A person is suffering from both nearsightedness and farsightedness. His spectacles would be made of
A.Two convex lenses with the upper lens having a larger focal length than the lower lens
B.Two concave lenses with the upper lens having a smaller focal length than the lower lens
C.A concave lens as the upper lens and a convex lens as the lower lens
D.D. A convex lens as the upper lens and a concave lens as the lower lens

Answer 1

Hint: Near sightedness or myopia means near objects clearly but cannot see distinct objects clearly. It is corrected by using a concave or diverging lens.
Hypermetropia or farsightedness is when distinct objects are clearly visible but nearby objects are difficult to see.it is corrected by a convex or diverging lens
Both such problems can be corrected using a combination of convex and concave lens.

Complete answer:
The combination of such lenses can be used in the spectacles of the person as the person is suffering from both myopia and hypermetropia.
In case of near sightedness, we use a concave lens which is called a minus power lens because it focuses the light from a point in front of the retina backwards so it falls on the surface of the retina which is necessary for image formation and processing in the eye.
Hence his spectacles would be made of a concave lens as the upper lens and convex lens as the lower lens.
In case of farsightedness, we use a convex lens or converging lens which helps light focus on the retina which is falling behind the retina in such a way it corrects hypermetropia or far sightedness.

Hence option C is correct.

Note:
Diverging lens or concave lens have negative focal length
Converging or convex lenses have positive focal length.
Presbyopia is another visual problem. Presbyopia is the normal loss of near focusing ability that occurs with age. It is a gradual thickening and loss of flexibility of the natural lens inside your eye.