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Human Eye
1.0Components and Structure of Human Eye
(1) The eye has a nearly spherical shape. The eye ball has a diameter of about one inch (nearly 2.3 cm). The front portion is more sharply curved and it is covered by a thin, transparent, protective membrane called ‘cornea’. This portion is visible from the outside.
(2) Behind the cornea, there is a liquid called ‘aqueous humour’ and behind that, there is a crystalline ‘lens’. Between the aqueous humour and the lens, there is a muscular diaphragm called ‘iris’, which has a small hole in it called ‘pupil’.
(3) The eye lens is composed of a fibrous, jelly like material which is hard in the middle and gradually becomes soft towards the edges. The curvature of the lens is altered by the ‘ciliary muscles’ to which it is attached.
(4) The space between lens and the retina is filled with another liquid called ‘vitreous humour’.
(5) The light entering the eye forms an image on the ‘retina’ which is a delicate membrane having enormous number of light-sensitive cells. It contains about 125 million receptors called ‘rods’ and ‘cones’ which receive the light rays and about one million optic nerve fibres which transmit the information to the brain.
(6) The region on the retina where the ‘optic nerve’ enters the eye ball is called the ‘blind spot’. It is insensitive to the light, that is, any image formed at this spot is not visible.
(7) The macula lutea, also called ‘yellow spot’ is the central part of the retina responsible for sensing fine detail and for looking straight ahead. It has high concentration of nerve endings and it is slightly raised. Its function is to form a very clear image, by sending a large number of electrical signals to the brain.
(8) ‘Sclera’ or ‘sclerotic’ is the outermost covering of eye and it is made of white fibrous tissue. ‘Choroid’ is a grey membrane attached to sclera.
2.0Working of Eye
(1) When the light enters the eye from air, most of the bending of light occurs at cornea. Some additional bending is done by the lens so as to form an inverted, real image of the object on retina.
(2) When the eye is focused on a distant object, the ciliary muscle relaxes allowing ligaments to increase tension on the lens and cause it to flatten i.e., the lens becomes thin or less curved. In this case, the focal length of the eye lens has its maximum value which is equal to its distance from the retina. The parallel rays coming into the eye from the distant object are focused on the retina and we see the object clearly.
(3) When the eye is focused on a closer object, the ciliary muscle contracts, allowing the lens, by virtue of its elasticity, to become more curved i.e., the lens becomes thick. In this case, the focal length of the eye lens decreases. The ciliary muscles adjust the focal length in such a way that image is formed on the retina and we see the object clearly.
Outer coat of the eye called sclera is white. It is tough so that it can protect the interior of the eye from accidents.
(4) 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 i.e., the brain makes an inverted image formed on the retina again erect
(or upright).
3.0Role of Iris
Iris controls the size of the pupil and therefore, helps in regulating the amount of light entering the eye through a variable aperture (the pupil). In low intensity of light, iris expands the pupil to allow more light to enter into the eye. When the light is very bright, iris contracts the pupil and the pupil becomes very small, thus, only a small amount of light enters into the eye.
The iris is that part of the eye which gives it its distinctive colour. When we say that a person has green or brown eyes, we refer to the colour of the iris.