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When do we consider a student sitting in the class to be myopic? List two causes of this defect. Explain using a ray diagram how this defect of eye can be corrected.
When a student sitting in the class is not able to read clearly the writing on the blackboard (or, unable to see any far away object), then we consider the student to have myopia (near-sightedness, or short-sightedness).
The two causes of this defect are:
1. High converging power of the eye lens, (because of its short focal length).
2. Eye-ball being too long.
This defect of eye can be corrected by using spectacles containing concave lenses.
A ray diagram given below illustrates how this defect of the eye can be corrected.
Explanation
Myopia, also known as near-sightedness or short-sightedness, is a defect of vision in which a person can't see the distant object clearly (appears blurred), though can see the nearby objects clearly. The far point of a myopic eye is less than infinity.
This defect occurs either due to the high converging power of the eye lens, (because of its short focal length). Or, due to the eye-ball being too long, which causes light to focus in front of the retina, instead of directly on the retina.
It is corrected by using spectacles containing concave lenses, which should be of such a focal length (or power) that it produces a virtual image of the distant object (lying at infinity) at the far point of the myopic eye.
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