The Human Eye and the Colourful World

The human eye

We are able to see things with the help of our eyes. The Eye is one of the most important sense organs. Let us see the structure of our eye.

The Shape of the eye is roughly spherical with an average diameter of around 2.3 cm. The outer part of the eye is quite tough and white in colour. This white part of the eye is known as sclera. The transparent, front outer covering of the eye is known as the cornea. Behind the cornea, there is a colored membrane known as the iris. It regulates the amount of light entering the eye. It also gives colour to the eye. In the iris, there is a variable sized, black circular opening known as the pupil. Its size is controlled by the iris. It appears to be black in colour because most of the light entering it is absorbed by the tissues, which are present in the pupil.

The size of the pupil depends on the brightness of light. It opens and closes in order to regulate and control the amount of light entering the eye. When we enter a dimly lit room, it takes the iris some time to expand the pupil to allow more light to enter the eye. For this reason, it takes us a few seconds to clearly see objects in a dimly lit room

Behind the pupil there is a lens which is thicker at the centre. It is made up of living cells. Two Ciliary muscles hold the lens within the eye-ball. The eye lens being convex in nature converges the light rays’ incident on it. Hence, it focuses the light falling on it on a thin layer of nerve cells called the retina. The retina is made up of a large number of nerve cells. Light falling on these nerve cells stimulate two kinds of sensitive cells known as cones and rods. Rods are sensitive to low light levels. Cones are sensitive to bright light, but they sense colours. Sensation felt by them is transmitted to the brain in the form of electrical signals through the optic nerve. This allows us to see.

The point where the retina and the optical nerve meet each other is devoid of any sensory cells. Hence, vision is not possible from this point. This point is known as the blind spot.

Take a white sheet of paper and write the alphabets ‘A’ and ‘Z’ on it (as shown in the give figure). Make sure that both alphabets are separated by atleast 8 cm. Now, close your right eye and look continuously at ‘Z’. Simultaneously, move the paper sheet slowly towards your eye. You will observe that the letter ‘A’ disappears at some point. What does this indicate?

It indicates that there exists a spot on the retina where no images are formed. Perform the same activity by closing your left eye and looking at letter ‘A’. This time the alphabet ‘Z’ would disappear. This implies that the blind spot is situated rightward in the right eye and leftward in the left eye.

The natural tendency of the iris and the pupil to contract and expand respectively, when exposed to bright light is used to check an unconscious person. Paramedics use this by shining a torch light in the eyes of an unconscious person to observe whether his/her iris or pupil is showing any change or not.

Persistence of an image

The image formed on the retina persists for aboutof a second. This means that if you are shown still pictures of a moving object at a rate faster than 16 pictures per second, then the object will appear to be moving. This is because, the image of a picture stays on your retina for of a second and you will not be able to recognize the time taken to change these pictures. This method is used in motion pictures where a large number of pictures are flashed at a rate of 24 images per second! Hence, they appear to be moving.

Do You Know:

Animals use their eyes in a special way. Crabs have very small eyes, which are located on the head. This helps a crab to look behind. Butterflies have a large number of eyes. An Owl’s eye is composed of a large numbers of rod cells and a very few number of cones on the retina. Hence, it is not able to see in daylight.

Do you know what happens if the image of an object does not form on the retina of an eye?

One will not be able to see clearly. The retina consists of photosensitive cells, which sends electrical pulses to the brain via the optic nerve. This enables us to see and sense objects.

Eye defects

We can see distant objects as well as the objects near us. The minimum distance up to which an eye can see clearly and distinctly without any stress is called the least distance of distinct vision.

The least distance of distinct vision for a normal eye is 25 cm.

The least distance of distinct vision varies as we grow older or because of some disease. This leads to many eye defects. For example, some people are able to see distant objects clearly; however they face problems in looking at objects close to them. On the other hand, some people can clearly see objects close to them, but face problems in looking at distant objects. These eye defects can be corrected by using suitable lenses (convex lens for the first defect, and concave lens for the second defect).

Sometimes with the passing of age, the eye lens can become cloudy and opaque. Due to this, the person’s eyesight becomes foggy. This defect is known as cataract. In this defect, a white spot can be seen in the eye lens. This …