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Human Eye

The bulged cornea, aqueous humour and the lens together produce refraction (Figure 9.24). The iris (diaphragm) regulates the amount of light entering the eye. Retina has light sensitive cells called cones.


The ability of the eye lens to adjust its focal length so as to obtain a clear image on the retina is called accommodation. However, the focal length of the eye lens cannot be decreased below a certain minimum limit. To see an object comfortably and distinctly, we must hold it at about 25 cm from the eyes, which is called the least distance of distinct vision/near point of the eye.

Defects of Vision

When the eye cannot clearly focus the image on the retina, the eye is said to have a defect of vision.

Details About the Defect, Its Cause and Remedy




1. Myopia (or near sightedness or short sight)

Eye ball is too long or the focal length of eye lens is short.

Description: UNFIG-9.7.tif 

Image is formed in front of retina.

Using a diverging lens of focal length equal to the distance of far point of the defective eye.

Description: UNFIG-9.8.tif 

Concave lens is used to correct myopia

2. Hypermetropia (or far sightedness or long sight)

Eye ball is too short or the focal length of the eye lens is large.

Description: UNFIG-9.9.tif 

Image is formed behind retina

Using a converging lens of focal length (1/f) = (1/v) + (1/u)

where u = d = 25 cm and v = near point of defective eye (O).

So, the rays from N appear to come from ‘O’.

Description: UNFIG-9.10.tif 

Convex lens is used to correct hypermetropia.

3. Presbyopia: The near point is farther than 25 cm and the far point is closer than infinity.

This is normally noticed in aged people.

The eye lens loses its flexibility with ageing.

A normal eye at the age of 10 years can change (its power range by about 4D) (from 25 cm to ).

At the age of 70 years, this range is only 1 D.

Use two separate spectacles, one with concave lens for distant objects and other with convex lens for nearby objects (reading etc). OR, use spectacles with bifocal lens.

4. Astigmatism:

The eye cannot clearly focus horizontal and vertical lines simultaneously.

The cornea has uneven curvature (non-spherical)

Use eye glasses with lenses having cylindrical curvature.


5. Cataract: Objects at all distances appear blurred and cloudy.

The eye lens has become less transparent (milky) due to ageing or injury.


The eye lens is removed by operation. A substitute lens is provided either inside the eye or outside, in front of it.

6. Night blindness:

Objects cannot be seen in dim light.

Inefficient working of cells in the retina due to deficiency of vitamin A.

During early stages, can be cured by intake of vitamin A.

Extending the Range of Vision

Angular Magnitude (Visual Angle) The angle subtended by an object at the eye is called the visual angle (α).


Description: Description: 89121.png

Figure 9.26


Visual Acuity Visual acuity is the angle subtended at the eye by a small object which is just visible to the eye. For a normal eye it is about 1 minute of an arc, that is, Description: 83306.png.
This is roughly equal to Description: 83318.png mm size at 25 cm (distance from the eye).

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