Name the three common defects of vision. What are their causes ? Name the type of lens used to correct each of them.

The Human Eye and the Colourful World

Defects of Vision and Their Correction

Official Answer

The three common refractive defects of vision, their causes, and corrections are as follows:

Myopia (Near-sightedness):

Causes: (i) Excessive curvature of the eye lens, or (ii) Elongation of the eyeball.
Correction: Corrected by using a concave lens of suitable power.

Hypermetropia (Far-sightedness):

Causes: (i) The focal length of the eye lens is too long, or (ii) The eyeball has become too small.
Correction: Corrected by using a convex lens of appropriate power.

Presbyopia:

Causes: It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens, usually associated with aging.
Correction: Corrected by using convex lenses (for near vision) or bi-focal lenses (if the person suffers from both myopia and hypermetropia).

MyopiaHypermetropiaPresbyopiaExcessive curvatureElongation of eyeballFocal length too longEyeball too smallCiliary musclesConcave lensConvex lensBi-focal lenses

Marking Scheme

1Naming the three defects: Myopia, Hypermetropia, and Presbyopia. (1 mark)
2Stating the specific causes for each of the three defects. (1 mark)
3Identifying the correct corrective lens for each (Concave, Convex, and Bi-focal/Convex). (1 mark)

Hint

Recall the three defects mentioned in Section 10.2. Think about whether the eyeball is too long or too short, and whether the lens needs to diverge or converge light to fix the image on the retina.

Quick Oral Answer

The three common defects are Myopia, Hypermetropia, and Presbyopia. Myopia is corrected with a concave lens because the eyeball is too long or the lens is too curved. Hypermetropia is corrected with a convex lens because the eyeball is too short or the focal length is too long. Presbyopia happens in old age due to stiffening of the lens and is corrected with convex or bi-focal lenses.

Analysis & Explanation

According to the NCERT text, vision becomes blurred when the eye loses its power of accommodation due to refractive defects.

In Myopia, the image of a distant object is formed in front of the retina. A concave lens diverges the incoming rays so they focus exactly on the retina.
In Hypermetropia, light rays from a nearby object are focused at a point behind the retina. A convex lens provides the additional converging power required to bring the image onto the retina.
Presbyopia is a condition where the near point recedes due to age-related loss of lens flexibility. While it often requires a convex lens for reading, bi-focal lenses (concave at the top for distance, convex at the bottom for reading) are used if the person also has myopia.

Common Mistakes

1Confusing the causes: thinking myopia is caused by a small eyeball instead of an elongated one.
2Swapping the corrective lenses: using convex for myopia and concave for hypermetropia.
3Forgetting that Presbyopia is specifically linked to aging and the weakening of ciliary muscles.

Interesting Facts

A normal eye can see objects clearly between 25 cm and infinity.

Bi-focal lenses were traditionally the only solution for combined defects, but today contact lenses and surgical interventions like LASIK are also common.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2011?

This question carries 3 marks in the CBSE Class 10 Science 2011 examination.

Which chapter does this question come from in Science?

This question is from the chapter "The Human Eye and the Colourful World" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Defects of Vision and Their Correction" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2011 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2011 paper.