Question

How is short sightedness (myopia) caused? How can it be corrected? Explain it with a diagram.

Answer 1

Hint: As we know there are mainly four common types of defects in vision. Which can be corrected by use of suitable eyeglasses. These defects are:-
Myopia or nearsightedness.
Hypermetropia or far sightedness.
Presbyopia.
Astigmatism.

Complete answer:
Firstly we will have a basic insight on myopia.
Myopia or short sightedness.
It is a vision defect in which a person can see nearby objects clearly but cannot see the distant objects clearly beyond a certain point .This defect is mainly common in children.
Now let see cause of myopia:
The defect arises due to either of the following two reasons:
The eyeball gets elongated along its axis so that the distance between the eye lens and the retina becomes longer.
The focal length of the eye lens becomes too short due to excessive curvature of the cornea.
As a result of the above cases, the parallel rays coming from a distant object do not meet at the retina shown in figure.
         

We can see from the above figure that the distant object is not seen clearly; the object has to be moved closer to the eye to a point F to focus it on the retina, as shown in the figure. Thus, the far point of the myopic eye is not at the infinity but only a few meters from the eye.
Now we will see correction of myopic eye
Correction of myopia:
A myopic eye is corrected by using a concave lens of focal length equal to the distance F from the eye. Concave lens diverges the parallel rays from distant objects as if they are coming from the far point F. Finally, we can see that the eye lens forms a clear image at the retina.

Additional information:
Long sightedness:
Long-sightedness may be additionally commonly referred to as hypermetropia or hyperopia is a common vision disorder. It results from the general power of the attention not being strong enough, or the length of the attention being too short.

Note:
The basic difference between myopia and hypermetropia is that a myopic person is merely ready to see the near or short distance objects clearly whereas a hypermetropic person is merely ready to see distant objects clearly. Laser eye surgery, clear lens exchange and surgical correction may be a treatment option for hypermetropia.