Write a short note on defect of vision.
Answers
Answer:
There are mainly three common refractive defects of vision. These are (i) myopia or near-sightedness (ii) Hypermetropia or far – sightedness (iii) Presbyopia. Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.
Answer:
Defects of vision is the loss of power of accommodation of the human eye. They are mainly of three types:
Myopia/ near-sightedness: The person can see nearby objects clearly but is unable to see objects beyond some distance. For such a person, far point changes to infinity to some finite distance. It is corrected with the help of a concave lens. The power of the lens is set so that its focal length is equal to the far-point of the eye.
Hypermetropia/ far-sightedness: The person can see far objects clearly but is unable to see nearby objects clearly. For such a person, near point increases to a distance greater than 25 cm. It is corrected with the help of a convex lens. The power of the lens is set so that the image of an object kept at 25 cm from the lens is formed at near-point of the eye.
Presbyopia: This defect is common in old-people and is caused by the weakening of ciliary muscles. In this defect, both the near-point and the far-point are affected. It is corrected with the help of a bifocal length which is a combination of convex lens and a concave lens.
