The Human Eye and its defects
The Human eye works on the refraction of the light through a natural convex lens, which is made up of transparent living material and enables us to see things that are present around us. And the ability to see is known as vision, eyesight or drishti. The human eye consists of the iris, pupil, cornea, ciliary muscles, optical never, retina and eye lens.
Construction of Eye
The front part of the eye is called cornea, which is made of transparent substance, and its outer surface is in a convex shape. The light coming from the objects enters our eyes through the cornea. Iris is located just behind the cornea, which is also called as a coloured diaphragm. The pupil is a hole in the middle of the iris. Behind it is the eye lens which is convex. The eye lens is held in position with the support of ciliary muscles. The eye lens is flexible and thus can change its focal length and shape with the help of ciliary muscles.
Behind the eye, the lens is retina on which the image is formed.
Working of the Eye
The light rays coming from any object enter our eyes through the pupil and falls on the eye lens. The eye lens then converges the light rays and produce an image of the object on the retina, which is real and inverted. Retina has a large number of light-sensitive cells which can generate electrical signals. Once the image is formed on the retina, it sends electrical signals to the brain, and we have a sensation of image. Also, even though the image formed on the retina is inverted out mind interprets it as erect.
So, the eye lens is the convex lens and retina is the screen of the eye.
The function of iris and pupil
The purpose of the iris is to adjust the size of the pupil. If the amount of light entering our eye is large, then pupil contracts and if the amount of light entering our eye is less, then pupil expands so that more light can enter the eye.
The adjustment of the size of a pupil takes some time, and this is the reason when we enter a dark room after coming from outside, we see things clearly after some time, or when we go out in the sunlight from a dark room, we feel glare in our eyes.
How do we see the colours?
The light-sensitive cells in the retina of our eye are of two shapes; rod shape and cone shape. The function of rod-shaped cells is to respond to the brightness of the light. And the purpose of cone-shaped cells is to make us see and distinguish colours.
Seeing distant and nearby objects
- Distant objects: When the rays of light, which are coming from a distant object, they are diverging at the beginning but later, become parallel when they reach our eye. Therefore to see a distant object, we need to have a convex eye-lens which has low converging power that focus them to form an image on the retina of an eye. The convex eye-lens of low converging power has a large focal length and is thin.
- Nearby objects: When the light rays are coming from the nearby object, they diverge when they are reaching our eyes. Therefore, to see a nearby object, we need to have a convex eye-lens with high converging power to focus and form an image on the retina. Convex eye-lens with high converging power has a short focal length and is thick.
Defects of vision and their corrections
There are three defects of vision, which are:
1) Myopia (Short-sightedness or Near-sightedness)
2) Hypermetropia (Long-sightedness or Far-sightedness)
3) Presbyopia
Myopia
The defect of an eye where it cannot see the distant objects is known as myopia. A person with myopia can see nearby objects. Myopia is caused due to:
- High converging power of a lens
- Eyeball being too long
The image which is formed in front of the retina due to high converging of the eye-lens and a person cannot see clearly the far objects. In another case, if the eyeball is too long than the retina is at a larger distance from the eye-lens. The image is formed in front of the retina even though the eye-lens has correct converging power.
Short-Sightedness or Myopia or can be corrected by wearing spectacles containing a concave lens. This is because when a concave lens of suitable power is used for the myopic eye, concave lens first diverges the parallel rays of light coming from a distant object. Therefore, first, a virtual image is formed at the far point of the myopic eye. Now since the rays of light appear to be coming from the eye’s distant point, they are easily focussed by the eye-lens and image is formed on the retina. A concave lens is used for a myopic eye to decrease the converging power of the eye-lens.
The formula for calculating the power of the concave lens to correct myopia is:
1/image distance (v)-1/object distance (u) = 1/focal length (f)
Hypermetropia
Hypermetropia, also known as long-sightedness, is a defect of an eye where a person cannot see nearby objects clearly. The near point of a hypermetropic eye is more than 25 cm away. This defect of the eye is caused due to:
- Low converging power of eye-lens
- Eyeball being too short
In the case of hypermetropia, the image of an object is formed behind the retina and therefore, a person cannot see nearby objects.
The near point of an eye having hypermetropia is more than 25 cm. The condition of hypermetropia can be rectified by using a convex lens in front of the eye. This is because when a convex lens of suitable power is placed in front of the hypermetropic eyes, convex lens first converges the diverging rays of light which are coming from a nearby object at the near point of the eye at which the virtual image of the nearby object is formed. Since the light rays now appear to be coming from the eye’s near point, the eye-lens can easily focus and form the image on the retina. A convex lens is used for hypermetropia to increase the converging power of the eye-lens.
Correction of Hypermetropia: The convex lens forms a virtual image of the object (lying at average near point N) at the near point N’ of this eye.
The formula for calculating the power of the convex lens to correct hypermetropia is:
1/v – 1/u = 1/f
In this formula, object distance that is u is normal near point of the eye (25 cm).
Presbyopia
This defect of vision usually happens in old age when ciliary muscles become weak and can no longer adjust the eye-lens. The muscles become inflexible in this condition and cannot see nearby objects clearly.
The near point of an older person having presbyopia is much more than 25 cm. Presbyopia can be corrected by wearing spectacles having a convex lens.
Another point to be noted is that a person can have both myopia and hypermetropia. In such a condition, spectacles having a bifocal lens are worn. The upper part of the bifocal lens is concave, and the lower part consists of a convex lens.
Cataract:
Another defect of the eye which usually comes in old age is the cataract. The medical condition in which the lens of the eye of a person becomes progressively cloudy, which results in a blurred vision. It develops when the eye-lens of a person becomes cloudy due to the formation of a membrane over it. It decreases the vision of the eye gradually and can lead to a total loss of vision of the eye. It can be restored after getting surgery. The opaque lens is removed, and an artificial lens is inserted in its place via operation. Any spectacle lenses can not correct this defect.
Power of accommodation of the eye
Accommodation is the ability of the eye to focus distant as well as nearby objects clearly on the retina of the eye is called.
The ciliary muscles are relaxed when our eyes see distant objects, then, and the focal length is maximum in this position. The eye-lens then converge the parallel rays of light to form an image of the distant object on the retina. When the eye sees the distant object, they are said to be unaccommodated.
And when our eyes see nearby objects, then the ciliary muscles get stretched, and its focal length decreases. Due to this, the converging power of the eye lens increases and the diverging rays of light coming from object converge to form an image on the retina. When the eyes see the nearby object, they are said to be accommodated.
The power of accommodation of a normal eye that enables it to see clearly an object is as close as 25 cm and as far as at infinity.
Why do we have two eyes for vision?
- Two eyes give a wider field view of 180 degrees.
- Two eyes help judge the distance of an object more accurately.
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