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Home
Science
Human Eye

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.

Human eye

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.

4.0Also Read

Scattering of Light

Conservation Of Linear Momentum

Power of Accommodation

Work, Energy and Power

Frame of Reference

Defects of Vision and Their Correction

Newton's Law of Motion and Frictions

Pulley

Refraction of Light Through A Prism

Table of Contents


  • 1.0Components and Structure of Human Eye
  • 2.0Working of Eye
  • 3.0Role of Iris
  • 4.0Also Read

Frequently Asked Questions

For a normal eye, the image is always formed at the retina whether it is viewing a closer or distant object. Thus, in both the cases, an image is formed at the retina.

In case (b), ciliary muscles are relaxed when the eye lens is thin and eye is viewing a distant object.

The change in the curvature of eye lens changes its focal length. When we look at a distant object, the ciliary muscles are relaxed, the lens becomes thin and the focal length of the eye lens is more (about 2.3 cm). Finally, the image is formed on the retina and we can see the distant object clearly. When we look at objects closer to eye, the ciliary muscles contract, so that the eye lens becomes more rounded in shape i.e., becomes thick. The focal length of the eye lens decreases so as to bring light from the nearby objects into focus on the retina.

The pupil of an eye acts like a variable aperture whose size can be varied with the help of the iris. When the eyes are exposed to bright light, the iris contracts the pupil to allow less light to enter the eye i.e., the size of pupil is reduced in the bright light. When you enter from bright light to a room with dim light, less amount of light enters in your eyes due to reduced size of the pupil. Thus, you are not able to see objects clearly for some time. After some time, iris expands the pupil due to dim light and allows more light to enter the eye and you are able to see things in the room. The pupil opens completely in dark/dim light through the relaxation of the iris.

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