By contracting or relaxing the ciliary muscles connected to the lens, its shape can be changed such that we can see the nearby as well as the distant objects clearly. This process is called ‘power of accommodation’ or ‘accommodation’.
The ability or property of the eye to change the shape of the lens so as to see the object clearly is called ‘accommodation’.
The muscles cannot be strained beyond a limit and thus, an object placed too close to the eye cannot be seen clearly.
In general, young children have a greater power of accommodation because they can clearly see objects that are very close to them. As we age, the lens in our eye becomes less flexible and its ability to accommodate decreases until we are forced to use corrective eye lens.
The nearest point for which the image can be formed clearly on the retina, is called the ‘near point of the eye’.
The minimum distance at which objects can be seen clearly without strain is called ‘least distance of distinct vision or clear vision’. In other words, the distance of the near point from the eye is called the ‘least distance of distinct vision’.
This distance varies with age; it increases with it because of decreasing effectiveness of the ciliary muscles and the loss of flexibility of the lens. The symbol used for least distance of distinct vision is D. Standard value of D for a young adult with normal vision is 25 cm. For a child of 10 years D is nearly 7 to 8 cm, for an old man of
60 years D is nearly 200 cm.
1. Move this page slowly towards your face. At a certain position of the page you will observe that the letters of this page just begin to blur.
2. Hold the page at this position and ask your friend to measure the distance between the page and your eye using a scale. This distance from the book to your eye is your near point.
The farthest point up to which the eye can see objects clearly is called the ‘far point’ of the eye. For normal eye, far point is at infinity.
The retina of a chick has mostly cones and only a few rods. As the cones are sensitive to bright light only, the chicks wake up with sunrise (dawn) and sleep by the sunset (dusk).
Some people do not possess some cone cells that respond to certain specific colours only. This is a genetic disorder called ‘colour blindness’. The persons who cannot distinguish between certain colours but can see well are called ‘colour blind’.
The defect of eye due to which a person cannot distinguish between certain colours is called ‘colour blindness’.
Driving license for vehicles is not issued to colour blind persons. Usually, colour blind persons cannot distinguish between red and green colours.
The crystalline lens of some people in old age becomes hazy or even opaque due to development of membrane over it. This defect is called ‘cataract’ which leads to decrease or loss of vision of the eye. Cataract can be removed by performing surgery to restore clear vision.
(Session 2025 - 26)