Question
State causes of myopia. How is this defect corrected? Draw suitable diagram to show it.
✓
Answer
- Causes of myopia:
- $(1)$ The focal length of the eye lens is decreased due to excessive curvature of the eye lens.
- $(2)$ The length of eyeball is elongated (longer than normal), i.e., the distance between the retina and the eye lens is increased.
- Due to either or both the causes, the eye lens is not able to focus the rays coming from a distant object (at infinity) on the retina.
[Corrected eye]
- Correction of the defect:
- To correct a myopic eye, the person has to wear spectacles with a concave (i.e., divergent) lens of suitable focal length or power.
- So that parallel rays coming from infinity appear, after refraction through the lens, to come from the far point of myopic eye, when the eye is unaccommodated. (figure)
- These parallel rays from a distant object are brought to the focus upon the retina with help of a concave lens.
- Obviously, the focal length of the required lens is equal to the distance of the far point from the eye.
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