Question

Describe myopia with the help of a ray diagram. Draw a ray diagram for the correction of myopia defect in the eye and explain the correction of myopia.

Accepted Answer

Myopia means short-sightedness. The defect of the human eye in which a person can see the objects lying at short distances clearly but cannot see the far objects distinctly is called myopia. A myopic child sitting on the back benches in the class cannot read what is written on the blackboard but can read the book comfortably. Thus, we can also say that a myopic person has no problem in reading and writing, but he cannot see clearly the objects beyond a particular distance. Thus, the far point of an eye suffering from myopia is less than infinity as shown in the following figure

In a myopic eye, the image of a distant object is formed in front of the retina, not at the retina itself as shown in the following figure.

Causes of myopia:

The defect myopia may arise due to

(i) Decrease in the focal length of the eye lens

(ii) The size of the eyeball becomes too long

Correction:

To correct a myopic eye, the person is allowed to wear spectacles with a concave lens of suitable power or focal length as shown in the following figure:

When a concave lens or diverging lens of suitable power is placed in front of the myopic eye, then the parallel rays of light coming from the distant object are first diverged by the concave lens. Thus, the concave lens forms a virtual image of the distant object at the far point of the myopic eye which can be easily focused by the eye lens to form an image on the retina. The concave lens used for correcting myopia should be of focal length that it produces a virtual image of the distant object at the far point of the myopic eye. The main object of using a concave lens is to reduce the converging power of the eye lens. The focal length of the concave lens used for correcting the myopic eye is equal to the distance of the far point of the myopic eye. Knowing the focal length of the concave lens, its power can be calculated.

Describe myopia with the help of a ray diagram. Draw a ray diagram for the correction of myopia defect in the eye and explain the correction of myopia.

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