Class 10 > Unit # 13: Geometrical Optics > Long Sightedness and Short Sightedness

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Geometrical Optics - Defects of Vision and Correction

Define short-sightedness and long-sightedness. Write reasons for these defects and explain how they are corrected?

1. Short-Sightedness or Myopia

"The defect of vision in which a person cannot see distant objects clearly, but can see clearly the objects near to him, is called short-sightedness or Myopia."

Reason for Myopia:

The reason for myopia is either that the focal length of the eye lens is too short or the eyeball is too much elongated. Consequently, light rays coming from a distant object are focused in front of the retina instead of directly on it.

Figure: Reason for Myopia.

Correction of the Defect:

This defect can be corrected by using a concave lens of suitable focal length, which diverges incoming light rays before they enter the eye so that they focus precisely on the retina.

Figure: Correction for Myopia.

2. Long-Sightedness or Hyperopia

"The defect of vision in which a person cannot see objects clearly which are near to him, but can see clearly distant objects, is called long-sightedness or hyperopia."

Reason for Hyperopia:

The reason for hyperopia is either that the focal length of the eye lens is too long or the eyeball is too short. Consequently, light rays coming from a near object are focused behind the retina.

Figure: Reason for Hyperopia.

Correction of the Defect:

This defect can be corrected by using a convex lens of suitable focal length, which converges incoming light rays slightly before they enter the eye so that they focus properly on the retina.

Figure: Correction for Hyperopia.

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