The human eye is a unique organ. It is the window to the world, enabling us to see the forms and colours of the world around us. The eye is one of the five sense organs of the human body. A pair of eyes is present on the head which allows us to see. But how does the eye see? Let’s find out some interesting facts about the human eye and colourful world.
Structure of the human eye
Image Credits: Exam Fear
The eye is almost spherical in shape. It has a diameter of average 2.3 cm. It has the following parts:
- Eyelids – external to the eye.
- Ciliary muscles
- Nerve cells
- Optic nerve
Cornea – The cornea is the white clear frontal membrane of the eye. It bulges outwards and is convex in shape. It protects the internal contents of the eye from injury as it is tough. The maximum refraction or bending of light happens at the cornea. The iris lies just behind the cornea.
Iris – the iris is the muscular diaphragm which allows light to enter the eye. The iris controls the amount of light entering the eye. It adjusts the size of the pupil and it is controlled involuntarily. The color of the iris is what is known as eye color – black, brown, blue, green etc.
Pupil – The pupil is that part of the iris which regulates the amount of light entering the eye by contracting or expanding in size. It is essentially an opening in the middle of the iris. Appears black as it absorbs light.
Lens – The lens is a biconvex colorless crystalline structure made of protein. It is transparent and projects the inverted real image of the outside objects on the retina.
Retina – The retina is the inner screen of the eye. It is light sensitive. Light converges on the retina to form an image. The retina is made of specialized nerve cells called rods and cones. Rods are sensitive to dim light. Cones are sensitive to bright light. Cones help in identifying colour. These nerve cells carry the information from the retina to the brain via the optic nerve.The brain interprets the image.
Aqueous humour – Humour in Latin means liquid. Aqueous humour is a watery clear fluid that is present in the space between the cornea and the lens. Aqueous humour provides nourishment to the lens and the cornea both.
Ciliary muscles – These muscles control the shape of the lens. Their contraction and relaxation determine the focal length of the lens.
Suspensory ligament – The ligament supports the lens and the ciliary muscle keeping them in position.
Vitreous humour – It is a clear dense jelly like fluid which fills up space behind the lens. The bulk of the eye is filled with vitreous humour and it imparts shape to the eye. Vitreous humour also helps to refract light on to the retina.
Optic nerves – The nerve located in the back of the eye. It exits the eye to travel upto the brain carrying visual information from the retina to the brain.
Blind spot – This is the contact point of the optic nerve and the retina. There are no sensory receptors here.
How does the eye work?
The working of the eye and the process of vision is similar to a camera.
Light enters the eye through the cornea and is reflected towards the pupil. The reflected light from the pupil then passes through the crystalline lens. The lens projects a real inverted and small image on the retina. In the retina, this light energy is converted via the rods and cones into electric signals which the optic nerve transmits to the brain for interpretation. This is how we are able to see the outside world around us.
The iris and the pupil control the amount of light entering the eye by dilation and contraction. The pupil dilates in dim light to absorb more light, and contracts in bright light to absorb less light. This function is protective in nature.
Accommodation is the ability of the eyes to adjust the lens in the eye to be able to perceive both near and distant objects and visualize clear and sharp images. This is accomplished by changing the focal length of the lens as and when required depending upon the distance of the object from the eye. When light from distant objects enter the eye, the ciliary muscles relax pulling the suspensory ligament tight thus effectively flattening the lens into a long and thin shape. When light from nearby objects enter the eye, the ciliary muscles tighten, suspensory ligaments relax and the lens becomes fatter and shorter.
This is the point nearest to the eye at which the object is visible distinctly. It is about 25 centimetres for the normal eye.
This is the maximum distance to which the eye can see normally. This is infinity for a normal eye.
Defects of the eye
Sometimes due to certain reasons, the eye cannot function normally. The three commonest such conditions are:
- Myopia/shortsightedness/nearsightedness –The image of a distant object is not formed on the retina but in front of the retina. Thus, such a person can see only the nearby objects clearly and distant objects appear blurred or hazy. Corrected using a concave or diverging lens.
- Hypermetropia/farsightedness – When the image is formed behind the retina, the person can see distant objects clearly but near objects blurred. Corrected using a convex or a converging lens.
- Presbyopia – Caused by aging. The image of the nearby objects forms behind the lens, due to the weakening of ciliary muscles and reduced lens flexibility.
- Cataract – This is caused due to loss of transparency of the lens by degeneration of lens proteins. Causes loss of vision. Corrected by removing old lens and replacing with an artificial lens.
We hope you had a comprehensive understanding of how an eye functions. If you want to know more about how animals respire, visit here.