Question

Far point of a person is 5 cm ,what type of spectacles should he use
a)concave lens with a focal length 5m
b)concave lens with focal length focal length 10m.
c)convex lens with focal length 5m.
d)convex lens with focal length 2.5cm

Answer 1

convex lens with focal length 2.5

Answer 2

Answer:

a)concave lens with a focal length 5 cm.

Explanation:

The far point of a person is that distance from the eye of the person at which an object is placed, along the optical axis of the eye, so that its image will formed on the retina of the person's eye.

If the far point of a person is 5 cm, it means that all the objects placed at 5 cm from the person's eye form respective images on the retina of that person.

So, the focal length of the lens to be used should be f = -5 cm.

The negative sign is taken as per the sign conventions used, the object is always taken on the left side of the lens which is taken negative.

The negative focal length means the lens is concave lens.

Thus, the correct option is (a) concave lens with a focal length 5 cm.

Note that the focal length is in cm not in m.