How The Human Eye Works
Human eye is the most delicate and complicated natural optical instrument which enables us to see the wonderful world of light.
Diagram shows the section of a human eye by a horizontal plane. It is a spherical ball of diameter about 2.5 cm. The eyelids act as the shutters of the eye and protect them from injury. Internally the eye is made up of the following parts.
- Cornea: It is the front buldged out part of eye ball covered by transparent sclerotic. It is a thin transparent tissue that covers the front of the eye.
Cornea of the eye-front view.
- Iris: It is the coloured part of the eye and is involved mainly in controlling the size of the pupil. Its colour differs from person to person. The iris increases and decreases the size of the pupil to regulate the amount of light that enters through the pupil.
- Sclera The white part of the eye that we can see is known as the sclera. It is filled with a clear watery fluid.
- Pupil: The inner aperture that we can see in the centre of the eye is known as the pupil. Pupil is like a hole through which light enters the eye. It is central circular aperture in the iris. Its normal diameter is 1 mm but it can contract in excess light and expand in dim light, by means of two sets of involuntary muscular fibres.
- Crystalline lens: It is a double convex lens L immediately behind iris. This is made of transparent concentric layers whose optical density increases towards the centre of the lens.
- Ciliary muscles: The lens is connected of the sclerotic by the ciliary muscles. These muscles change thickness of the lens by relaxing and exerting pressure. These control the focal length of the eye lens.
- Aqueous humour: Anterior chamber is filled with a transparent liquid of refractive index. The liquid is called the aqueous humour.
- Vitreous humour: Posterior chamber is filled with a transparent watery liquid with little common salt having some refractive index. The liquid is called the vitreous humour.
- Retina: Just behind the eyeball is a lining called the retina. It is the retina that is sensitive to light and has receptors called rods and cones. These rods and cones respond to light and generate impulses that can be read by the brain. The brain then sends back messages that tell us what we have seen. It forms innermost coat in the interior of the eye. It consists of a thin membrane which is rich in nerve fibres, containing two kinds of vision cells called rods and cones and blood vessels. It is sensitive to light, for it is a continuation of the optic nerves. It serves the purpose of a sensitive screen for the reception of the image formed by the lens system of the eye. [The rods are responsible for colour vision in dim light (Scotopic vision). The cones are responsible for vision under ordinary day light (Photopic vision).
- Lens It is a transparent tissue between the pupil and the retina. The lens helps in focussing the light that passes through the pupil into the eye. This helps in focussing the image on the retina, by bending the light rays.
- Optic nerve: It connects the eye to the brain and carries impulses to and from the brain.
- Blind spot: The blind spot B. There is a portion on the retina where the nerve fibres enter the optic nerve. This portion does not have any rods and cones, and images falling on this portion of the retina cannot be ‘seen’. This spot is called the blind spot. It is the spot where the optic nerves enter the eye. It is also slightly raised and insensitive to light, because it is not covered with choroid and retina.
Working (Action of the eye):
The human eye is like a camera. Its lens system forms an image on a light-sensitive screen called the retina. Light enters the eye through a thin membrane called the cornea. It forms the transparent bulge on the front surface of the eyeball. The eyeball is approximately spherical in shape with a diameter of about 2.3 cm. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea. The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distances on the retina. We find a structure called iris behind the cornea. Iris is a dark muscular diaphragm that controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye. The eye lens forms an inverted real image of the object on the retina. The retina is a delicate membrane having enormous number of light-sensitive cells. The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.
When light rays reach the lens of the eye, passing through the pupil, they bend and an inverted image is focussed on the retina. The rods and cones of the retina convert the image into an electrical impulse, which is taken by the optic nerve to the brain. The brain interprets the impulse and we comprehend what we have seen.
The image produced in the human eye is retained for a very short period of time after the object (producing the image) is removed. This phenomenon is called persistence of vision. The disc appears white when it is rotated because the images of the different colours overlap in our eyes and the brain perceives it as white. Sometimes the lens becomes cloudy or opaque. This causes a condition called cataract. Cataract is a very common disease all over the world and causes blurred or dimmed vision. In some cases there could be double vision or no vision at all. Using corrective glasses or contact lenses can rectify minor cases of cataract. Laser rays or even surgical operations can treat major ones. Cataract affects millions of people all over the world. Whenever there is any visual difficulty, we should consult an eye specialist immediately.
We can ‘see’ clearly only if the image is formed (focussed) on the retina. If the image is formed in front of the retina, or behind it, we cannot see clearly and it will appear to be blurred. For a person with normal eyes, it is easy to see both nearby and far off objects clearly. While reading, a book is held about 25 cm away from the eyes. However, you would have noticed that some people need to hold the book very close to their eyes to be able to see the writing clearly. Such people are said to be short sighted. On the other hand, there are some people who find it difficult to see things that are nearby but find it easier to see objects that are far away. For example, they may find it easier to read the writing on a billboard, but might find it difficult to read a book at a normal distance. Such people are said to be long sighted.
The formation of the image in front of the retina results in a condition called myopia or short sightedness. A person suffering from myopia cannot see far off objects very clearly. This defect can be corrected by wearing spectacles with concave lenses. The formation of the image behind the retina results in a condition called hypermetropia or long sightedness. A person suffering from hypermetropia cannot see nearby objects very clearly. This defect can be corrected by wearing spectacles with convex lenses.