The Human Eye Structure- Class 8 Science Guide

The Human Eye Structure- Physics Guide for Class 8

Information about The Human Eye Structure

Title	The Human Eye Structure
Class	Class 8
Subject	Class 8 Physics
Topics Covered	The Human Eye The Blind Spot How we see Colours?

The world around us is known to us largely through our senses. The sense of sight is very important among all our senses. The human eye, one of our most valuable and sensitive organs, gives us this sense of sight. The eye is perhaps the most important, and also most perfect, optical device that nature has endowed us with. It is the eye that enables us to see all the 'beauties and bounties' of nature all around us. We all know that this plays a very important and crucial role in our life. We use our eyes in activities like reading, writing, driving, observing nature and in countless other ways.

• We have already studied about the phenomenon of refraction of light and its role in the working of lenses.
• We have also studied about the nature, position and relative size of images formed by different types of lenses.
• We also know that it is only when light from an object enters our eye that we are able to see that object.
• These basic ideas will help us in our study of the human eye. 

The Human Eye 

The human eye (eye ball) is nearly spherical in shape with a diameter of about 2.5 cm.

Cornea

• Light enters the eye through a transparent curved (tough) front surface. This is known as the cornea of the eye.
• It is whitish in colour (the 'white' of the eye).
• Its main function is to act as a protective layer for the eye. 
• The space behind it is filled with a liquid called acqueous humor. 

Iris

• Behind the cornea is a dark coloured muscular diaphragm called the iris. 
• The iris may be pigmented and is responsible for the characteristic colour of the eye of a person.

Pupil

• A small circular aperture (opening) is present in the centre of iris. The size of this aperture is variable and self-adjustable. This aperture is known as the pupil. 
• The pupil appears black as no light is reflected from it. 
• The iris regulates the amount of light entering the eye by adjusting the size of pupil. 
• In dim light, the pupil gets enlarged and thus lets more light enter the eye. In bright light, the pupil contracts. 
• It is this self-adjustment of the size of the pupil that not only protects the interior of the eye from excessive brightness but also improves its image forming ability.

Eye Lens

• The light entering the eye is focussed by the eye lens.
• The eye lens is a convex lens made up of transparent crystalline layers. It is harder at its middle and gradually becomes softer towards its edges.
• The eye lens is held in its position by ciliary muscles. 
• These muscles help in changing the curvature and hence the focal length of the eye lens.

Retina

• The lens of the eye forms a real, inverted image of the object on the inner coat of the eye. This screen of the eye is called the retina. 
• It is a light sensitive screen.
• It is a delicate membrane having enormous number of light sensitive cells or photoreceptors.
• These light sensitive cells are of two types:
  (i) The rod-shaped cells
  (ii) The cone-shaped cells. 
• The rods and cones (sensory nerve cells) respond, respectively, to the amount of light energy and to the colours present in it.
• The rods are responsible for the vision in dim condition and the cones help us in colour vision.
• The cones get activated only in bright light conditions. 

Activity 1 

To demonstrate how light sensitive cells respond to low/dim light conditions.

• Take a collection of objects that look almost identical (e.g. caps of different colours but of same size). 
• First work in a bright room and separate out the caps into piles of similar colours. 
• Count the number of caps of each type. 
• Mix them together after doing the counting. 
• Now, turn off most of the lights so that the room is only very dimly lit. 
• Again try to separate out the caps into similar coloured piles. 
• Turn on the lights and look at the results. 
• Count the number of errors made by you. 
• Mix up the lot once again. 
• Dim the lights again and after waiting for 7-10 minutes, again try to separate the caps in the same dim light conditions. 
• Switch on the lights and again count the errors.

What do you observe? 

There would be fewer errors this time because the light sensitive cells have now got time to adapt themselves to the dim light conditions. 

The 'Blind Spot'

At the juction of the optic nerve and the retina, there are no 'rods' and 'cones' (sensory cells).

No image gets formed at this point as it is insensitive to light. This point is called the blind spot of the eye. 

We can demonstrate its existence through the following activity. 

Activity 2 

To demonstrate the existence of blind spot. 

• Draw a thick broken blue line and a red circle on a white sheet of paper. The distance between the line and circle may be adjusted to be about 6-8 cm. 
• Hold the sheet of paper at an arm's length from the eye. Close your right eye. 
• Look at the red circle and slowly move your head closer to the image.

What do you observe? 

At a certain distance the blue line will not look broken. This happens when the image of this 'broken line' falls on the blind spot of the retina. The brain fills up the 'missing information' and we see the broken line as a complete unbroken line.

You can even draw the following figure on a white sheet of paper. 

Observe at what distance the 'space', between the vertical lines, 'vanishes'. Now close your left eye instead of right eye. Repeat the above steps.

What do we conclude?

The blind spot position varies for the left eye and the right eye. For both the eyes, the blind spot is not exactly in the same place. 

How do we see colours?

We now know that our retina has a large number of light sensitive cells having shapes of rods and cones. 

• The rod shaped cells respond to the amount of incident light energy, i.e. to different degrees of brightness and darkness. 
• In dim light, the rods are sensitive but the cones are not. 
• They (the cones) respond mainly to the colours of the incident light. 
• They become active in bright light only and then enable us to make colour perception possible. 
• The cones are generally sensitive to red, green and blue light to different extents. 

Some Important Points

• The cornea is the tissue in the human body which does not contain blood vessels.
• The eye muscles are the most active muscles in the human body. 
• Our eyes contain (nearly) 7 million cones which help us see the colours and the details of the object. Also, there are (nearly) 120 million cells, called rods, which help us to see better in dim light conditions. 
• Animals have eyes shaped in different ways. Eyes of a crab are quite small, whereas butterflies have large eyes that seem to be made up of thousands of little eyes. They can see not only in front and sides but towards the back as well. 
• A night bird (like owl) has more rod cells compared to cone cells. Also, this bird has a large cornea and a large pupil to allow more light to enter its eye. The night birds are, therefore, able to see very well even during night. They find it difficult to see during the day. This may be due to the 'rather large amount of light' that can enter through their 'large' pupil. The day birds (like kite, eagle, chick) have more cone cells compared to rod cells. Thus they can see very well during the day. The decrease, in the number of their rod cells, makes it difficult for them to see during dim light conditions, i.e. at 'night time'. 
• The octopus does not have a blind spot. The photoreceptors, in the retina of the octopus, are located in the inner part of the eye while the cells, that carry information to its brain, are in the outer portion of its eye. Its optic nerve, therefore, does not cross any point of its retina.
• Some people, due to some genetic disorder, do not possess some cone cells that can respond to certain specific colours only. Such persons, who cannot distinguish between colours, but can otherwise see well, are said to be colour blind. In some persons, the rod cells are fewer in number compared to cone cells. Such persons find it difficult to see clearly in dim light conditions. They are said to be affected by night blindness. 
• The colour perception of different animals is different due to different structure of their rod and cone cells. Not all animals are able to distinguish colours. Human beings, apes, monkeys, birds and some fishes are the only animals that are able to distinguish colours well. Most of the domestic animals are colour blind. The bees can perceive some colours which we cannot do. This is because bees have some cones that are sensitive even to ultraviolet light.