The human eye is an organ that reacts to light and allows vision. Human eye is also called camera-type eye which allows light perception, colour and depth perception. A human eye is roughly 2.3 cm in diameter and is almost spherical in shape filled with some fluid.
A human eye has following main parts:
- Sclera: It is an opaque, fibrous, protective ,outer layer of an eye containing collagen and elastic fibre. It is also known as white of the eye.
- Cornea: It is the transparent spherical membrane covering the front of the eye. Light enters the eye through this membrane.
- Iris: It is dark muscular diaphragm between the cornea and the lens. It controls the size of the pupil. It is the colour of the iris that we call the colour of the eye.
- Pupil: It is the small hole between the iris through which light enters the eye. In dim light, it opens up completely due to contraction of iris muscles, but in bright light it becomes very small due to relaxation of iris muscles. It controls the amount of light that enters the eye.
- Lens: Behind the pupil, there is a transparent, biconvex structure called a lens. It facilitates the image formation. By the action of Ciliary muscles, it changes its shape to focus light on the retina. It becomes thinner to focus distant objects and becomes thicker to focus nearby objects.
- Retina: It is the light-sensitive surface of eye on which the image is formed. It converts images formed by the lens into electrical impulses. These electrical impulses are then transmitted to the brain through optic nerves.
- Optic Nerves: It transmits visual information from the retina to the brain. Optic nerves are of two types:
Rods– These are the light sensitive cells. They help in peripheral vision. These cells are more sensitive to dim light.
Cones– These are photoreceptor cells. They help in detailed central and colour vision and are more sensitive to bright light.
- Blind spot: At the junction of the optic nerve and retina. It contains no rods and cones, so no vision is possible at that point.
- Conjunctiva: It is a thin transparent membrane that is spread across the sclera. It keeps the eye moist and clear by secreting mucus and tears.
- Aqueous Humour: It is a watery fluid that is present in the area between the lens and the cornea. It is responsible for the nourishment of both the lens and the cornea.
- Vitreous Humour: The space between eye lens and retina is filled with a semi-solid, transparent, jelly-like substance called vitreous humour. It plays an important role in maintaining the shape of the eye and also causes refraction of light before it reaches the retina.
DEFECTS OF HUMAN EYE
1- Myopia (Near-Sightedness)
- One cannot see distant objects.
- The image is formed before the retina.
- A concave lens is used for the correction of this defect.
- The power of the lens is -ve.
2- Hypermetropia (Far-Sightedness)
- One cannot see near objects.
- The image is formed behind the retina.
- A convex lens is used for the correction of this defect.
- The power of the lens is +ve.
3- Astigmatism
- In this defect, the curvature of the cornea becomes irregular.
- The image is not clear.
- A cylindrical lens is used for the correction of this defect.
4- Presbyopia
- In this defect, the power of accommodation of lens decreases due to age.
- A bifocal lens is used for the correction of this defect.
- It is also known as age sightedness.
5- Cataract
- In this defect, the lens becomes opaque.
- It can be cured by surgery.
6- Glaucoma
- In this defect, the fluid pressure within the eye rises.
- The treatment involves surgery, laser treatment or medication.
7- Xerophthalmia
- It is a medical condition in which the eye fails to produce tears.
- This is also known as Night Blindness.
- It can ultimately lead to cornea damage.
- It is caused by the deficiency of vitamin A.
POINTS TO REMEMBER
1- The pupil appears black, because no light is reflected from it.
2- It is impossible to sneeze with your eyes open.
3- All babies are colour blind when they are born.
4- Cornea of shark has been used in eye surgery due to its similarity with human cornea.
5- Number of rod cells is greater than cone cells.
6- The image formed on the retina is real and inverted.
One reply on “HUMAN EYE AND ITS DEFECTS”
Thanks for another informative blog. Where else could I get that type of information written in such an ideal way? I have a undertaking that I’m simply now working on, and I have been at the look out for such information.