NCERT solutions for class 10 science The Human Eye and the Colorful World
Today we will learn ‘The defects of vision and their correction’.
Defects of vision and their correction
The ability to see is called vision.It is also called an eyesight.Vision is known as ‘Drishti’ in Hindi. There are three common defects of vision or defects of the eyes:
- Myopia
- Hypermetropia
- Presbyopia
Myopia (Short-sightedness or nearsightedness)
Myopia is the defect of vision due to which a person cannot see the distant objects clearly, though he/she can see the nearby objects clearly. The far point of an eye suffering from myopia is less than infinity.
The defect of the eye called myopia is caused:-
- Due to thehigh converging power of eye lens because of its short focal length.
- Due to the eyeball being too long.
In some cases, in an eye suffering from myopia, the ciliary muscle attached to the eye lens does not relax sufficiently to make the eye-lens thinner to reduce its converging power. So due to the greater converging power of the eye-lens in amyopic eye, the image of a distant object is formed in front of the retina and hence the eye cannot see it clearly.
Myopia is corrected using spectacles containing concave lenses. The concave lens used for correcting myopia should be of such a focal length that it produces a virtual image of the distant object at the far point of the myopic eye. The whole purpose of using a concave lens here is to reduce the converging power of the eye-lens.
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