Question

Eye is said to be nearsighted when a?

Answer 1

Hint: Eye is the main organ of sight. Nearsightedness is due to the refractive error in the eye. Refractive error is the ability of the eye to focus the image of objects accurately on the retina. Eye with normal refractive power is called an emmetropic eye and the condition is called emmetropia. Any deviation in refractory power is called ametropia.

Complete answer:
The eye consists of a lens that helps in focusing the image onto the retina. Due to the change in the refractive properties of the lens, the image is not directly focussed on the retina. The disease in the eye is majorly caused by refractive error. One of such diseases is myopia.
Myopia or short-sightedness: The word myo means half-closed and ops means eye. It generally occurs in children and in a few adults too.

Myopia is an eye defect characterized by the inability to see the distant object. It is otherwise called short sightedness because the person can see near objects clearly but not distant objects.
In myopia the near vision is normal but the far point is not infinite. This means it is at a definite distance. In extreme conditions, the far point may be only a few centimetres away from the eyes.

Now let's understand the cause of myopia.
The cause of myopia: the refractive power of the lens is usually normal but the anteroposterior diameter of the eyeball is abnormally long. Therefore, the image is brought to focus a little in front of the retina. Light rays, after coming to a focus, disperse again, so a blurred image is formed upon the retina.

Note: Correction of myopia, in myopic eye, in order to form a clear image on the retina, the light rays entering the eye must be divergent and not parallel. Thus, the myopic eye is corrected by using a biconcave lens. Light rays are diverged by the concave lens before entering the eye. Another defect of the eye is called hypermetropia or farsightedness.