Human Eye and Colourful World
The human eye
Have you wondered why the eye is able to focus the images of objects lying at various distances?
It is made possible because the focal length of the human lens can change i.e., increase or decrease, depending on the distance of objects. It is the ciliary muscles that can modify the curvature of the lens to change its focal length.
To see a distant object clearly, the focal length of the lens should be larger. For this, the ciliary muscles relax to decrease the curvature and thereby increase the focal length of the lens. Hence, the lens becomes thin. This enables you to see the distant object clearly.
To see the nearby objects clearly, the focal length of the lens should be shorter. For this, the ciliary muscles contract to increase the curvature and thereby decrease the focal length of the lens. Hence, the lens becomes thick. This enables you to see the nearby objects clearly.
The ability of the eye lens to adjust its focal length accordingly as the object distances is called power of accommodation.
The minimum distance of the object by which clear distinct image can be obtained on the retina is called least distance of distinct vision. It is equal to 25 cm for a normal eye. The focal length of the eye lens cannot be decreased below this minimum limit of object distance.
Let us see what happens when an object is at a distance less than 25 cm from the eye lens.
The far point of a normal eye is infinity. It is the farthest point up to which the eye can see objects clearly.
The range of vision of a normal eye is from 25 cm to infinity.
Have you ever thought why animals’ eyes are positioned on their heads?
This is because it provides them with the widest possible field of view. Our eyes are located in front of our face. One eye provides 150° wide field of view while both eyes simultaneously provide 180° wide field of view. It is the importance of the presence of two eyes as both eyes together provide the three-dimensional depth in the image.
The loss of power of accommodation of an eye results in the defects of vision.
There are three defects of vision called refractive defects. They are myopia, hypermetropia, and presbyopia. In this section, we will learn about these defects of vision in detail.
1. Myopia (short sightedness)
Myopia is a defect of vision in which a person clearly sees all the nearby objects, but is unable to see the distant objects comfortably and his eye is known as a myopic eye. A myopic eye has its far point nearer than infinity. It forms the image of a distant object in front of its retina as shown in the figure.
Myopia is caused by
increase in curvature of the lens
increase in length of the eyeball
Since a concave lens has an ability to diverge incoming rays, it is used to correct thi...
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