Definition
The eye is one of the five sense organs of human beings that are equipped to receive light and convert it to visual impulse.
Human beings have binocular vision, i.e. vision is created by both the eyes perceiving a single three-dimensional image of the object when light reflected from it falls on the retina and the impulse is carried by the optic nerve to the brain where it is processed. The eyes sit in two bony cavities called eye sockets, protected by the upper and lower eyelids and eyelashes.
The Basic Structure of the Human Eye
The human eye consists of the following structures:
The Three Coats: The human eye is surrounded by three coats, the sclera, the choroid and the retina. The sclera is the fibrous outermost layer which is white and opaque in the posterior 5/6th portion. The anterior 1/6th portion of the sclera is transparent and is known as the cornea. While the posterior part ensures that light entering the eye does not pass through, the anterior portion allows the entry of light into the eye.
The choroid is the middle layer which is supplied with blood vessels. The anterior part of the choroid forms the annular pigmented iris. The opening of the iris is known as the pupil of the eye. Ciliary bodies behind the iris remain attached to the lens by suspensory ligaments. They together help in the attachment and in changing the shape of the lens.
The innermost layer is known as the retina which is provided with photosensitive cells. These cells are rod and cone cells which are responsible for receiving dim and bright light respectively. Cone cells are also sensitive to colour and help in coloured vision. The central portion of the retina where light entering perpendicular to the eye has the highest concentration of cone cells and is called the macula lutea. The centre of the macula lutea is called fovea centralis or yellow spot and is responsible for the brightest and sharpest vision.
The Spaces in the Eye: The space between the iris and the cornea is filled with a fluid called the aqueous humor and the space between the retina and the lens is filled with vitreous humor. They help in maintaining the pressure and shape of the eyeball and also act as a refractive medium of the eye.
Lens: The lens of the human eye is a transparent bi-convex structure which acts as the main refractory medium focusing the light on the retina. A basement membrane called the lens capsule surrounds the lens. The main bulk of the lens is made up of lens fibres. The anterior side of the lens has a layer of lens epithelium between the lens capsule and the outermost layer of the lens fibre.
How is the Image of an Object Formed by an Eye?
Or if framed alternatively, the question would be how are we able to see an object? – The answer is explained below with the help of a schematic diagram:
The image of an object is formed on the retina situated in the inner eyeball when light rays reflected from that object converge at the cornea after passing through the lens. Light rays from the topmost point and bottom of the object in question are traced to the eye where they produce an inverted real image right on the retina.
Facts About the Eye
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The human eye is capable of distinguishing between 10 million different colours with three different types of cone cells. These cone cells are capable of distinguishing between red blue and green light.
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The eyes, unlike nose ears and skin, do not grow and remain the same size.
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The human eye blinks 4200000 times a year and is one of the fastest muscles of the human body.
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The most common colour of the iris of the eye in the world is brown and not black. Over 55% of people in the world have this eye colour. Brown eye colour is actually blue underneath.
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The cornea of the eye does not have any blood vessel. Presence of blood vessels would interfere with vision.
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The fear of eyes is known as ommetaphobia, also known as emetophobia.
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Human beings and sharks have similar cornea and hence, the shark’s cornea can be used to replace human cornea.
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The nerves of the eyes and nose are connected to the brain by cranial nerves as a result of which when we sneeze the stimulus travels to the eyelids. This results in the closing of the eyes while sneezing. We cannot sneeze with our eyes open.
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Tobacco smoke is responsible for causing dry eyes.
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Diabetic retinopathy is the condition where the blood vessels are blocked due to long term diabetes. 40% of the people suffering from type 1 diabetes suffer from this condition.
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The blinking of eyes is much less in babies than in adults.
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The lens loses its elasticity and its ability to focus on age. As a result, 99% of people need a reading glass between the age of 43 to 50.
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When a person’s pupils are not of the same size from birth, the condition is known as anisocoria.
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The nerve cells in the retina and the brain are not fully developed in babies. Hence they do not perceive colour, they see objects in black and white at birth.
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Men and women see colours differently as shown by several research works probably due to the male hormones testosterone which affects the way the brain processes the information differently.
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In the case of eye surgery, the replacement of the cornea is possible. However, entire eye transplants are not possible.
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Tigers have six times better eye vision than human beings
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There are more than one million nerve cells in the optic nerve.
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Learning comes through the eyes 80% of the time.
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The iris is more unique than fingerprints. It has 256 unique characteristics than our fingerprints.
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The corneal scratches take only 48 hours to heal.
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The image formed by the eyes is inverted. They are processed and converted back to an erect image by the brain.
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There is a blind spot (or scotoma) in the eye which is the region from which the optic nerve arises. It is also known as the optic disc. This is the region which is completely devoid of any rod or cone cells as a result of which light falling on this spot does not create any image. Hence is the name. The missing information on the blind spot is filled by the brain with information from the surrounding.
The human eye is a super-specialised sense organ that endows us with vision. It reacts with light and allows colour vision, light vision, and depth vision.