The Human Eye and the Colourful World

Human Eye

  • The human eye works on the principle of refraction of light through a natural convex Lens of transparent living material and enables us to see things around us
  • The main parts of the human eye are : Cornea, Iris, Pupil, Ciliary muscles, Eye lens,Optic nerve

Construction of the Eye

  • Eye lid An eye-lid in front of the eye which is just like the shutter in a camera.
  • When eye-lid is open, light can enter the eye but when eye-lid is closed, no light enters the eye.
  • Cornea- The front part of the eye is called cornea. It is made of a transparent substance. and it is bulging outwards
  • The outer part of the cornea is convex in shape
  • Pupil – there is a hole in the middle of the Iris which is called pupil of the eye
  • The pupil appears black because there is no light is reflected from it

  • Eye Lens- The eye-lens is a convex lens made of a transparent, soft and flexible material like a proteins
  • Being flexible, the eye-lens can change its shape (it can become thin or thick) to focus  the retina.
  • The eye-lens is held in position by suspensory ligaments

  • Ciliary muscles change thickness of eye-lens while focusing.
  • In other words, the focal length of eye-lens can be changed by changing its shape by the
  • In this respect an eye differs Iris from a camera. The focal length of the convex lens used a camera is fixed and cannot be changed but the focal length the convex lens present inside the eye can be changed by the action of ciliary muscles.
  • Retina The screen on which the image is formed in the eye called retina.
  • The retina is behind the eye-lens and at the back part of the eye.
  • The retina of an eye is just like the film in a camera.
  • The retina is a delicate membrane having a large number of light sensitive cells called ‘rods’ and ‘cones’ which respond to the ‘intensity of light’ and colour of objects’ respectively, by generating electrical signals.
  • Blind spot is a small area of the retina insensitive to light where the optic nerve leaves the eye
  • When the image of an object falls on the blind spot, it not be seen by the eye.
  • Aqueous humour’-The space between cornea and eye-lens is filled with a watery liquid called ‘aqueous humour’.
  • Vitreous humour- And the space between eye-lens and retina is filled with a transparent jelly like substance called ‘vitreous humour’ which supports the back of the eye.

Why We Could not See for Some time when entering from outside to a dark room.

  • The eye is small (as in a dark room or during night), the iris expands the pupil (makes the pupil large) so that more light may enter the eyes
  • Thus, the iris regulates (or controls) the amount of light entering the eye by changing the size of the pupil.
  • If the intensity of the outside light is low, then the pupil expands to allow more light to enter the eye
  •  This Process Takes some time to happen
  • If the amount of light received by the eye is large (as during the day time), then the iris contracts the pupil (makes the pupil small) and reduces the amount of light entering the eye

Rods and Cones

  • The retina of our eye has a large number of light-sensitive cells.
  • There are two kinds of light sensitive cells on the retina : rods and cones
  •  Rods are the rod-shaped cells present in the retina of an eye which are sensitive to Intensity of light
  •  Rods are the most important for vision in dim. light (as during the night). We can see things to some in a dark room or in the darkness of night due to the presence of rod cells in the retina of our eye
  • Cone Cells helps to detect color of the object

Working of the Eye

  • The light rays coming from the object kept in front of us enter through the cornea of the eye, pass through the pupil of the eye and fall on the eye-lens.
  • The eye-lens is a convex lens, so it converges the light rays and produces a real and inverted image of the object on the retina. (Actually, the outer surface of cornea also acts as a convex lens due to which cornea converges most of the light rays entering the eye. Only the final convergence of light rays is done by the eye-lens to focus the image of an object exactly on retina).
  • The image formed on the retina is conveyed to the brain by the optic nerve and gives rise to the sensation of vision.
  • Actually, the retina has a large number of light-sensitive cells. When the image fall on the retina then these light-sensitive cells get activated and generate electrical signals.
  • The retina sends these electrical signals to the brain through the optic nerve and gives rise to the sensation of vision.
  • Although the image formed on the retina is inverted, our mind interprets the image as that of an erect object.

Eye and a Camera 

The human eye is like a camera. In the eye, a convex lens (called eye-lens) forms a real and inverted image of an object on the light-sensitive screen called retina whereas in a camera, the convex lens (called camera-lens) forms a real and inverted image of an object on the light sensitive photographic film.

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