Sources of Light
Light is the most important source to see any object. We need light this is a part of our daily life. We may have a keen sight and the object we want to see may be big. But we can see it in light only. To make objects visible to us we need a source of light. We see an object when light from some source falls on the object, gets scattered and reaches our eyes. We get the sense of sight of that object. Thus light enables us to see objects.
Sources of light: An object that can give light is called source of light. The sun is the most important source of light for us. It is the most brilliant source of light. Although it is about 150 million kilometers from the earth, it sends us light during the daytime. The sun is a natural source of light. So are the stars. They are not man-made. Some stars are more brilliant than the sun, but only twinkle in the sky at night because they are too far away from us. Glow-worms are other natural source of light. You may have noticed them during the night time. They look very beautiful at night. The light emitted by them is too feeble. Apart from natural sources of light, there are some man made artificial sources of light. Wax-candle, oil-lamp, torch, electric bulb etc are some of them. The intensity of light depends on the source of light. For example, a candle produces the intensity that is better than that of a glow-worm. An electric bulb produces much more light than a candle. The intensity of light produced by a source of light is compared with that of a candle. The units are called Candle power.
The electric bulb used in our houses possesses 100 candle-power or more. That means it can produce the same intensity of light from 100 candles burning at a time. At noon the intensity of light from the sun is about a million candlepower or more. The modern unit of intensity of light, accepted internationally, is Lumen. One lumen is equivalent of 12.56 candle power. Nearly 700 lumens is equal to one Watt.
There are many sources of light give out heat also. While the electric bulb burns, it gives out heat also besides giving us light. A tube-light gives out less heat when compared to an electric bulb of same wattage. The sun gives us light as well as heat. Glow-worms are cool sources of light. You can catch some glow-worms and feel about it yourself. The light given by agarbathi will be about the same as that of a glow-worm. If the temperature of a glow-worm reached the temperature at the burning end of the agarbathi, the worm would simply die. Some sources of light are cool while some are hot. The reason is the method by which light is produced in the source. An electric bulb has a wire in it called the filament. The filament glows when it reaches very high temperature. electricity is sent into the filament to heat it. The glowing filament is the burning filament. Therefore an electric bulb is hot and you cannot touch it when it is giving out light. But in a tube-light a different mechanism is used to produce light. Therefore the tube remains cool when it gives our light. You may find out this for yourself. Don't touch an electric bulb when it is giving out light. It will be very hot. In a glow worm the energy produced during a chemical reaction in its body is released in the form of light only-not as heat. Therefore a glow-worm is a cool source of light. The sun is a big burning sphere. The reactions taking place in the sun are very powerful. The energy produced in those reactions is released in the form of light energy and heat energy also. Therefore the sun and stars are sources of light and heat. A burning candle, a glow-worm, lamps and lights and the sun are sources of light. They give out light by themselves. Such sources of light are called Self-luminous bodies.
Trees, books, the earth etc, are not visible in darkness. They cannot produce light. Such bodies that cannot produce their own light are called Non-luminous bodies. The moon is also a non-luminous body. It reflects the sun's rays of light. It is a cool non-luminous mirror. The natural sources of light in the sky are the sun and the stars only. The planets and the moon are only reflectors of light. Just like moonlight we can also say about the earthlight. The light rays from the sun are reflected by the earth. This reflected light is called the earth light. The astronauts who landed on the moon watched the
earthlight from there.
Oil lamps or kerosene lamps: People used oil seeds to light lamps. Later on, they started using kerosene lamps instead of oil from seeds. A wick is put in oil and lighted. The oil first vaporizes owing to heat. This vapour mixes with oxygen in the air and burns and this becomes the flame. The flame gives out heat and light. As the oil at the tip of the wick vapourise, the oil at the bottom rises up through capillaries in the wick. This process continues and the tip of the wick goes on burning.
Kerosene vaporizes faster and easier than oil. Therefore in hurricane lamps and petromax lights kerosene is used.
The Hurricane Lamp :
There are four important parts in a hurricane lamp.
(1) A small metal tank to pour kerosene
(2) Burner
(3) Wick for air to enter
(4) Glass chimney.
At the bottom of the lamp there is a small kerosene tank. The wick is fixed into the burner. One end of the wick is put inside the kerosene in the tank. The other end is lighted. Kerosene goes up by capillary action through the fine holes in the wick and burns at the top end of the wick. The hot air produced inside the chimney goes out through the holes at the top of the lamp. The holes in the burner let the cool outer air enter the chimney for the wick to burn continuously. To prevent blowing winds from putting out the lamp, a glass chimney is provided. The chimney also helps in distributing the light evenly in all directions.
The Electric Bulb :
This was invented by Thomas Alva Edison, an American scientist. Because it gives out light when electricity is sent into it, it is called the electric bulb. There is a thin wire made of tungsten in a bulb. This is called the filament. The two ends of this filament are attached to two copper wires whose ends are fixed to two terminals at the top of the bulb. The air in the glass bulb is removed and sealed. Otherwise the oxygen in the air will burn out the filament and it will not last even a few hours. The air is removed and inert gases like nitrogen or helium is filled into the bulb and sealed. The filament lasts a long time and also gives more light. When electricity is sent through the copper wires, it passes through the filament. Then the filament gets heated, glows and gives out light.
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