5.1 How can we see ?

During the day time, we can see things clearly. But we switch on the lights to see them at night. If we close our eyes we cannot see anything even during day time or at night with lights on. So, let us find what requirements should be fulfilled to have "vision".

Discuss with your friend about the differences of the observations taken down when the hole was closed and when the torch light was directed in.

From the above activity, you can understand that light is needed to see something clearly.

Is only light enough to see an object ? Are there any other needs to be completed to see an object clearly ?

Even though you close your eyes, the hand still gets light. Then, why didn’t you see the lines on the palm which you observed when your eyes were opened? To have a good vision, not only light but also the eye is important. The main factors which are needed to see are a source of light and the eye.

5.2 Sources of Light

The main natural source of light is the sun. At night, we use bulbs, lamps, candles and other artificial sources to get light because there is no sunlight. Other than that, we get light even from the moon and stars at night.

We get light from these objects because they emit light. Such objects which emit light are known as sources of light.

Luminous and Non Luminous Objects

We can’t see anything when we enter a dark room. But if we light a bulb or a candle we will be able to see the candle and the bulb as well as other objects in the room. The reason is that the light emitted by the candle or the bulb enters our eye. The objects which can produce their own light is called luminous objects.

You may have observed organisms like glow-worm and firefly produc- ing their own light. There are also some types of mushrooms which emit light.

Think of the same dark room which was mentioned before. We are able to see not only the bulb but also other things such as bed, chair and table after the blub was lit. But if the bulb was not lit, we won’t be able to see them. The reason for this is that those objects do not emit light.

Such objects which do not emit light are known as non luminous objects. To get a vision of such an object, a light ray of a luminous object must fall on it.

We can see the bright moon in the night sky. Because of this you must have thought that it is a luminous object. But the moon does not emit its own light. We see it as a bright object because the rays of light emitted by the sun fall on the surface of the moon. So, we consider it as a non luminous object. Not only the moon but also the planets are non luminous too.

5.3 Transmission of Light through Objects

Glass is used in windows of many houses to get light into the house. But sometimes curtains are used to limit the amount of light fallen in. In some bathrooms decorative glasses are used to get light.

We use the property of light transmission through objects in different ways. Let us investigate further on this.

Record your observations in the following table.

Now according to the observations, categorize the objects into three groups as follows.

Decide to which group (A,B,C) do the objects used in the above activity belong. Accordingly, fill in the following table with the relevant objects.

We used only solid objects for the above activity. But, the way light transmits through liquids differs from other. Light transmits through air very easily and therefore, we can see the things around us clearly. But, when there is fog in the atmosphere, it is not possible to see things clearly.

On the other hand, we can observe different coloured fish in a tank with clear water. But, if the water is dirty and unclear, we cannot see fish clearly. This happens because light travels in different ways through water and air. Let us do the following activity to investigate how light travels through liquids.

When we look through some liquids, we can see both the objects and the light clearly. The reason is that light is transmitted well through such liquids. They are called transparent objects. Glass and transparent colourless polythene are examples for transparent objects.When we look through oil papers, unclear water, coloured cellophane papers, although the light can be seen, the objects cannot be clearly seen. Such objects are known as translucent objects.

When we look through most of the objects we cannot see either the light or the object. For example, cardboard, wood, metal, paper, tar and etc. They are known as opaque objects.

Now you can identify the objects in the table categorized under A, Band C as transparent, translucent and opaque objects respectively.

5.4 Light Rays and Light Beams

You may have observed the light filtering through clouds or trees on a foggy morning or light emitting from headlights of a vehicle on a rainy night. They are known as light beams. Let's do the following activity to learn about light beams.

When the string is in a straight line, we can observe the flame. But, when we change its position, we cannot observe the flame through the holes.

Here, you are able to observe the flame because the light travelled from the flame to the eye through the holes. When the holes are in a straight line, the flame could be observed and when they are not in a straight line, flame couldn't be seen. Now, you can understand that light is transmitted only in a straight line.

In the above activity, the string which was put through the holes shows the path of the light ray.

The narrow path in which light is transmitted known as a light ray. To show a light ray, we use a straight line with an arrow.

A light beam is made up of a collection of light rays. You have seen the light beam which is emitted from a torch when it is switched on. We use several straight lines with arrow heads to show a light beam.

From Activities in 5.7 and 5.8, we can understand that light rays trans- mit in a straight line.

5.5 Applications of Light

Light is a major need for proper vision. With the development of tech- nology, we have changed the behaviour of light and obtained many uses.

Let us categorize the applications of light as follows.

For the production of food in plants

Unlike other living organisms, green plants can produce their own food. The energy needed for the process of food production is obtained from the sunlight.

You all know that all other animals directly or indirectly depend on plants for food. Because of this, we consider that production of food in plants is the main advantage of light.

Let us find more instances where we use these advantages of light.

To give a clear vision

Earlier, people received light from natural objects such as the sun, the moon and stars. But after the invention of fire, man was able to produce light from various sources.

With the development of technology, the light bulb was invented to get light. Today there are many types of bulbs in the market.

Illumination

At present, most of the countries in the world use different coloured and patterned bulbs to illuminate various occasions.

Large light sources are used in market places, populated streets and shopping complexes for advertisements. You must have seen that not only at night but also during the day time, the lights are kept on.

As signals

An instance where light is used to give signals is the light used in light houses. They release a powerful beam of light which gives the signal to the sailors that the land is close.

In traffic lights, you may have seen the colours red, green and yellow. They individually give a message to the drivers and pedestrians. More attention is gained through the use of light to give such signals. Red is a colour which shows a danger.

For communication

Communication in the modern world is well developed. For this, different types of light rays are used. Telephone messages are transmitted through optic fibers.Telephone connection networks across the main cities of Sri Lanka have been built using optic fibers. Optic fibers are used to connect computer networks and high standard audio visual equipment. They use optic fibers and laser rays to send telephone messages and to maintain internet connections.

In the medical field

At present, doctors use laser rays to treat eye patients, in heart operations and to treat wounds in the digestive system. This has increased the efficiency of treatments.

With the use of optic fibers, the interior parts of the body are easily observed. Here, the equipment used to lighten the interior of the body is called the endoscope.