The world is largely known through the senses. The sense of sight is one of the most important senses. Through it we see mountains, rivers, trees, plants, chairs, people and so many other things around us. We also see clouds, rainbows and birds flying in the sky. At night we see the moon and the stars. You are able to see the words and sentences printed on this page. How is seeing made possible?
Have you ever thought how we see the various objects? You may say that eyes see the objects. But, can you see an object in the dark? It means that eyes alone cannot see any object. It is only when light from an object enters our eyes that we see the object. The light may have been emitted by the object, or may have been reflected by it.
Fix a white sheet of paper on a drawing board or a table. Take a comb and close all its openings except one in the middle. You can use a strip of black paper for this purpose. Hold the comb perpendicular to the sheet of paper. Throw light from a torch through the opening of the comb from one side (Fig. 16.1). With slight adjustment of the torch and the comb you will see a ray of light along the paper on the other side of the comb. Keep the comb and the torch steady. Place a strip of plane mirror in the path of the light ray (Fig. 16.1). What do you observe?
Imagine that parallel rays are incident on an irregular surface as shown in Fig. 16.6. Remember that the laws of reflection are valid at each point of the surface. Use these laws to construct reflected rays at various points. Are they parallel to one another? You will find that these rays are reflected in different directions. (Fig. 16.7)
Recall the last time you visited a hair dresser. She/he makes you sit in front of a mirror. After your hair cut is complete, she/he places a mirror at your back to show you how the hair has been cut (Fig. 16.9). Can you think how you could see the hair at the back of your head?
You are aware that a plane mirror forms
only a single image of an object. What
happens if two plane mirrors in
combination are used? Let us see.
Take two plane mirrors. Set them at right angles to each other with their edges touching (Fig. 16.10). To hinge them you can use adhesive tape. Place a coin in between the mirrors. How many images of the coin do you see (Fig. 16.10)?
In Class VII, you learnt that the sunlight is referred to as white light. You also learnt that it consists of seven colours. Here is another activity (Activity 16.7) showing that sunlight consists of several colours.
We see things only when light coming
from them enters our eyes. Eye is one
of our most important sense organs. It
is, therefore, important to understand
its structure and working.
The eye has a roughly spherical
shape. Outer coat of the eye is white. It
is tough so that it can protect the
interior of the eye from accidents. Its
transparent front part is called cornea
Fig. 16.14). Behind the cornea, we find a dark muscular structure called iris. In the iris, there is a small opening called the pupil. The size of the pupil is controlled by the iris. The iris is the part of that eye which gives it its distinctive colour. When we say that a person has green eyes, we refer actually to the colour of the iris. The iris controls the amount of light entering into the eye. Let us see how.
It is necessary that you take proper care
of your eyes. If there is any problem
you should go to an eye specialist. Have
a regular checkup.
1. If advised, use suitable spectacles.
2. Too little or too much light is bad for eyes. Insufficient light causes eyestrain and headaches. Too much light, like that of the sun, a powerful lamp or a laser torch can injure the retina.
Some persons, including children, can be visually handicapped. They have very limited vision to see things. Some persons cannot see at all since birth. Some persons may lose their eyesight because of a disease. Such persons try to identify things by touching and listening to voices more carefully. They develop their other senses more sharply. However, additional resources can enable them to develop their capabilities further.
The most popular resource for visually challenged persons is known as Braille. The present system was adopted in 1932. There is Braille code for common languages, mathematics and scientific notation. Many Indian languages can be read using the Braille system.