Contents
Some of the most important Physics Topics include energy, motion, and force.
What are Sources of Light? And What are Luminous and Non-Luminous Objects ?
Light is a form of energy. Light is needed to see things. We cannot see an object in the darkness of night. We need a source of light to make the object visible. During the day, sun’s light allows us to see objects. At night, it is the light of a torch, an electric bulb or a tube-light which makes us see objects.
So, as soon as light from a torch (or an electric bulb) falls on the object at night, we are able to see it clearly. Thus, light enables us to see objects from which it comes (or from which it is reflected). We are able to see the beautiful world around us because of light.
We can read a book, see pictures in a magazine and watch TV and movies due to the existence of light. And it is light which makes us see our image in a looking mirror. We detect light with our eyes. We will now discuss the various sources of light.
Sources of Light
An object which gives out light is called a source of light. The various sources of light around us are: Sun, Stars, Electric bulb, Tube-light, Kerosene oil lamp, Candle, Torch and Firefly (Glow-worm).
The most important source of light for us is the sun. The sun is a very bright source of light. Even though sun is about 150 million kilometres away from us, its light makes the daytime bright on the earth. At night we use an electric bulb, tube-light, kerosene oil lamp, candle or torch as the sources of light.
The firefly (or glow-worm) is a very feeble source of light. Though firefly looks very pretty in the dark night, but its light is so dim that it is of no use to us. Firefly (or glow-worm) is called ‘jugnu’ in Hindi.
Natural and Man-Made Sources of Light
All the sources of light can be divided into two groups : natural sources of light, and man-made sources of light.
(i) Those sources of light which occur in nature as such are called natural sources of light. The sun is the best natural source of light. Nearly all the natural light we receive on the earth comes from the sun. The stars, meteors (shooting stars) and firefly are the other natural sources of light.
(ii) Those sources of light which have been made by man are called man-made sources of light (or artificial sources of light). Electric bulb, tube-light, kerosene oil lamp, candle and torch, are all man-made sources of light.
Luminous and Non-Luminous Objects
An object which gives out its own light is called a luminous object. The sun, other stars, burning candle, lighted electric bulb, lighted torch, glowing tube-light, television screen (when TV is switched on), a piece of red hot iron, the flame of a gas burner and firefly (jugnu) are all luminous objects. This is because they give out their own light. The luminous objects are, in fact, the sources of light.
Since luminous objects give out their own light. They are luminous objects, give out their own light, they can be seen even in the dark. A luminous object can be seen because the light given out by it enters our eyes.
For example, we can see the sun because the light given out by sun enters our eyes. Similarly, we can see an electric bulb at night (when it is switched on) because light given out by the glowing bulb enters our eyes.
An object which does not give out its own light is called a non-luminous object. The table, chair, book, book-rack, flower-pot, plants, trees, clothes, bag, carpet, mirror, camera, diamond, pen, pencil, a car bumper, painting, shoe, moon, earth and other planets, etc., are all non-luminous objects. This is because they do not give out their own light.
The non-luminous objects can be seen only when light coming from a luminous object (like the sun, an electric bulb, a tube-light or torch, etc.) falls on them. This light is reflected by non- luminous objects in all directions. When this reflected light enters our eyes, we can see the non-luminous objects.
Thus, we can see the non-luminous objects because they reflect light (received from a luminous object) into our eyes. For example, the moon is a non-luminous object which does not have its own light. We can see the moon because it reflects sunlight into our eyes.
Actually, when sunlight falls on the surface of moon, then some of this sunlight is reflected (sent) by the moon towards the earth. And to us it appears as if the light is being given out by the moon itself. But in reality, moonlight is the sunlight which is reflected from the surface of moon.
Thus, moon behaves as a cold, non-luminous mirror. It is a reflector of sunlight. Just like moon, the planets can also be seen in the sky because they are reflectors of sunlight. Let us now take another example.
A flower is a non-luminous object. It does not give its own light. We can see a flower because it reflects sunlight falling on it into our eyes (see Figure).
In Figure, the sunlight falls on the flower. The flower reflects some of the sunlight falling on it in all the directions. When the sunlight reflected by flower enters our eyes, we are able to see the flower (because to us the light appears to be coming from the flower itself).
During night time, the flower is visible to us because it reflects the light received from an electric bulb (or tube-light) towards us. In fact, we see most of the objects around us by reflected light. Most of the objects reflect daylight (or other light) falling on them. And when some of this reflected light goes into our eyes, we are able to see the object.
We can see many things kept in a room during the day on which sunlight does not fall directly.
This happens as follows : All the objects outside the room (which are in direct sunlight) scatter the sunlight falling on them in all the directions. Even the air scatters some sunlight. It is this scattered sunlight which enters the room and helps us to see the things lying in the room which are not in direct sunlight.
The table kept in our room is a non-luminous object which does not give out its own light. We can see the table because it reflects the light falling on it (from the sun, an electric bulb or a tube-light) in all directions. And when this reflected light coming from the table enters our eyes, we are able to see the table.
To us it appears as if the light is coming from the table itself. We cannot see an object if light from that object does not reach our eyes. For example, the light from an object lying behind a wall does not reach our eyes, so we cannot see an object lying behind a wall.