The Human Eye and its defects
The Human eye works on the refraction of light through a natural convex lens made up of transparent living material and enables us to see things around us. And the ability to see is called vision, eyesight or drishti. Human eye consists of cornea, iris, pupil, ciliary muscles, eye lens, retina and optical nerve.
Construction of Eye
The front part of the eye called cornea, is made of transparent substance and its outer surface is convex in shape. It is through cornea that the light coming from objects enters the eyes. Just behind the cornea is iris which is also called coloured diaphragm. A hole in the middle of the iris is called pupil. Then behind it is the eye lens which is a convex lens. It is due to the support of ciliary muscles that the eye lens is held in position. Eye lens is flexible and thus can change its focal length and shape with the help of ciliary muscles.
Behind the eye lens is retina on which the image is formed in the eye.
Working of the Eye
The light rays coming from the object enter the eyes through pupil and falls on eye lens. The eye lens then converge the light rays and produce an image of the object on retina which is real and inverted. Retina has large number of light sensitive cells which can generate electrical signals. After 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 retina is inverted out mind interprets it as erect.
So, eye lens is the convex lens and retina is the screen of the eye.
Function of iris and pupil
The function of iris is to adjust the size of the pupil. If the amount of light entering the eye is less then pupil expands so that more light can enter the eye and in case the amount of light entering the eye is large then pupil contracts.
The adjustment of the size of pupil takes some time and this is the reason when we go outside in the sunlight from a dark room we feel glare in our eyes or if we enter a dark room after coming from outside we see things clearly after some time.
How do we see 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 light. And the function of cone shaped cells is to make us see colours and distinguish between them.
Seeing distant and nearby objects
- Distant objects: When the rays of light are coming from distant object they are diverging at the beginning but become parallel when they reach our eye. Therefore to see a distant object, we need to have a convex eye-lens of low converging power to focus them to form an image on the retina of the eye. The convex eye-lens of low converging power has large focal length and is quite thin.
- Nearby objects: When the rays of light are coming from nearby object they diverge when they reach our eyes. Therefore, to see a nearby object we need to have a convex eye-lens of high converging power so as to focus and form an image on the retina. Convex eye-lens with high converging power has short focal length and is thick.
Power of accommodation of the eye
The ability of the eye to focus distant as well as nearby objects clearly on the retina of the eye is called accommodation.
When our eyes see distant objects then the ciliary muscles are relaxes 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 retina. When the eye see 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 retina. When the eyes see 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.
Defects of vision and their corrections
There are three common defects of vision. They are:
1) Myopia (Short-sightedness or Near-sightedness)
2) Hypermetropia (Long-sightedness or Far-sightedness)
The defect of an eye in which it cannot see the distant objects clearly is called myopia. A person with myopia can see nearby objects clearly. Myopia is caused due to:
- High converging power of lens
- Eye-ball being too long
Due to high converging of the eye-lens the image is formed in front of the retina and a person cannot see clearly the distant objects. In another case, if the eye- ball is too long than the retina is at larger distance from the eye-lens. In this case also the image is formed in front of the retina even though the eye-lens has correct converging power.
Myopia or short-sightedness can be corrected by wearing spectacles containing concave lens. This is because when a concave lens of suitable power is used for the myopic eye then the concave lens first diverge the parallel rays of light coming from 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 eye’s far point, they are easily focussed by the eye-lens and image is formed on retina. Concave lens is used for myopic eye so as to decrease the converging power of the eye-lens.
Formula for calculating power of concave lens to correct myopia is:
1/image distance (v)-1/object distance (u) = 1/focal length (f)
Hypermetropia or long-sightedness is a defect of an eye where a person cannot see nearby objects clearly. The near-point of hypermetropic eye is more than 25 cm away. This defect of eye is caused due to:
- Low converging power of eye-lens
- Eye-ball being too short
In case of hypermetropia the image of an object is formed behind the retina and therefore, a person cannot see clearly nearby objects.
The near-point of an eye having hypermetropia is more than 25 cm. The condition of hypermetropia can be corrected by putting 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, then the convex lens first converge the diverging rays of light 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 retina. Convex lens is used for hypermetropia so as to increase the converging power of the eye-lens.
Correction of Hypermetropia: The convex lens forms a virtual image of the object (lying at normal near point N) at the near point N’ of this eye.
Formula for calculating power of 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).
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 old person having presbyopia is much more than 25 cm. Presbyopia can be corrects by wearing spectacles having convex lens.
Another point to be noted is that a person can have both myopia and hypermetropia. In such a condition, spectacles having bifocal lens are worn. The upper part of bifocal lens is concave and lower part consists of convex lens.
A yet 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 resulting in 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 total loss of vision of the eye. It can be restored after getting surgery .The opaque lens is removed and artificial lens is inserted in its place via operation. This defect cannot be corrected by any type of spectacle lenses.
Why do we have two eyes for vision?
- Two eyes give wider field view of 180 degree.
- Two eyes help judge the distance of an object more accurately.