In the previous section we completed a discussion on the 'heating effect of electric current'. In this section, we will see the 'lighting effect'.
Consider the filament bulb shown in fig.10.3 below:
1. It has a metallic conductor known as the filament.
• This filament is supported between two copper wires
2. The current enters the filament through one of the copper wires.
• It then passes through the filament.
• After passing through the filament, the current comes out of the bulb through the other copper conductor.
3. We know that when current passes through a conductor, that conductor gets heated.
• In our present case also, the current is passing through a conductor, which is the filament. So the filament will surely gets heated.
4. But instead of the heat, here we are going to see light.
• This is because, when current passes through the filament, it becomes so hot that, it becomes white in color, and begins to emit heat.
5. But to produce light in this way, there is a condition:
• The filament should be made with a 'suitable material'. Then only it will emit light when current passes through it.
6. Most metals emit red light when they become red hot. But we want warm yellow or white light.
• Also the filament should last for a long time. Then only the bulb can produce light.
7. The great scientist and inventor Thomas Alva Edison tried nearly a thousand different materials before he could successfully make a filament.
■ Today, filaments are made using the metal tungsten.
8. Some precautions are to be taken to ensure the durability of the tungsten filament:
(i) When it is white hot, tungsten will react with the oxygen in the bulb. To avoid this, all the atmospheric air is evacuated from inside the bulb
(ii) Because of the high temperature, tungsten will gradually evaporate. To avoid this, nitrogen is filled into the bulb at a low pressure. Nitrogen is used because it is an 'almost inert' gas and is abundantly available.
9. Filament bulbs are also called incandescent bulbs.
• 'Incandescent' means 'emitting light as a result of being heated'
• We can write: In an incandescent bulb, electrical energy is converted to light energy and heat energy
■ Why don't we make filaments using nichrome?
• Reason 1: Nichrome emits red light when it becomes hot
• Reason 2: Nichrome is not as ductile as tungsten. So nichrome cannot be drawn into thin wires.
♦ We want the filaments to be made of thin wires
♦ These thin wires are made into coiled shapes.
♦ Coiled shape helps to accommodate greater length in lesser space.
■ Light up an incandescent bulb for some time. Turn it off and touch it carefully. We can feel that it is very hot.
• It is clear that a large portion of the electrical energy is lost as heat energy.
• In order to prevent such a heavy loss, many countries have now banned the use of incandescent bulbs
■ However, the heat produced by filament bulbs can be utilised in some special cases. Let us see how it is done:
1. When the filament is heated, we get light.
2. Along with the light, infrared radiations are also produced.
• These infrared radiations can be felt as heat.
3. So the 'spherical glass covering of the bulb' is tinted with red.
• This will reduce the visible light.
• We will thus obtain the heat alone.
• Such bulbs are used in poultry farms to keep the birds warm
Some images can be seen here.
4. The infrared radiations can be concentrated to a specified point by using special reflectors.
• The heat at such points can thus be increased to required values.
• Lamps with such reflectors are used in the field of medical treatment
Now we will see some other methods by which electricity can be used to produce light:
2. The tube is filled with noble gases like argon, neon, krypton and xenon under low pressure.
3. At each sealed end of the tube, an electrode is fitted. One is the anode and the other is the cathode.
Fig.10.4 below shows an image:
4. When the tube is connected to an electric circuit, some of the 'atoms of the gas near the anode' are ionized.
• That is., those atoms lose electons and become positive ions.
5. These positive ions (cations) are accelerated towards the cathode.
• During this travel, they collide with the ordinary atoms present in the gas.
6. When this collision occurs, the ordinary atoms are excited to higher energy levels.
• When these 'atoms at higher energy levels' return to their normal energy level, the excess energy is radiated as light.
7. The colour of the light thus produced, depends on the type of gas. It also depends on the pressure at which the gas is filled.
• Neon gives red-orange light • Argon gives violet light • Krypton gives green light • Xenon gives grey light • Sodium vapour gives yellow light
2. The tube is filled with mercury vapour.
3. At each end of the tube, a heating coil is fitted.
4. When the tube is connected to an electric circuit, the heating coils become very hot and they begin to emit electrons.
• Thorium oxide is coated on the heating coils to increase the 'electron emission efficiency'.
5. The rapidly moving electrons collide with the molecules of the mercury vapour.
• As a result of this collision, ultra violet rays are produced.
• But ultra violet rays is not visible light. So it does not serve the purpose of lighting the surroundings.
6. Now, the inside surface of the tube is coated with a fluorescent material.
• The ultra violet rays produced from the mercury vapour is absorbed by the fluorescent material.
7. The atoms of this fluorescent material are affected during this absorption:
• In what way are those atoms affected?
(i) Electrons present in the orbitals of these atoms are excited to higher energy levels.
• Thus the energy level of the atom as whole increases.
(ii) These atoms then try to attain stability by giving off the excess energy.
• For that, the excited electrons fall back to their original orbitals.
(iii) When they fall back, they give off the excess energy. This 'energy which is given off' is in the form of visible light.
8. Thus we see that, the visible light is actually produced by the 'fluorescent material'.
• But this fluorescent material needs a 'source'. The ultra violet rays from the mercury vapour is the source
9. The fluorescent lamp requires a very high voltage to get started.
• For that purpose, a ballast is used. The ballast serves another purpose also:
• It prevents excessive current from flowing into the tube
10. The initial cost of fluorescent lamps is greater than that of incandescent lamps.
• But there is a lesser loss of electricity in the form of heat.
• Also they give more light
• So fluorescent lamps are more economical than incandescent bulbs
2. Both contains mercury vapour.
3. The length of the tube is reduced in a CFL
4. The electronic ballast required for the proper working, is kept at the end of the tube.
• Fluorescent lamps in general contain mercury vapour
• If these lamps are improperly disposed off at the end of their useful periods, the mercury vapour will spread into the atmosphere. This will create health problems
• So we must all work together to create social awareness about this problem
2. When suitable current is passed through a specially designed circuit, some electrons are able to fall back into lower energy levels. When this happens, they give off energy in the form of light. This is the basis of LED lamps. We will learn more about them when we learn electronic circuits.
3. As there is no filament, there is no loss of energy in the form of heat
4. As there is no mercury vapour, it is not harmful to the environment
5. A small quantity of energy can give a large quantity of light
6. LED lamps can surely help us to fight the energy crisis.
2. Initially a high voltage is applied between the electrodes. This will cause the gas atoms to collide with each other.
3. Then they lose electrons and become ions.
4. When these ions return to their low energy state, the excess energy is radiated as light.
• The colour of the light depends on the type of gas filled in the tube.
5. Here also, the ballast serves two purposes:
• It produces the high voltage required to get started.
• It prevents excessive current from flowing into the tube
6. Arc lamps are used in search lights, movie shooting, flood lights, rescue works during night time etc.,
Consider the filament bulb shown in fig.10.3 below:
1. It has a metallic conductor known as the filament.
• This filament is supported between two copper wires
 | ||
| Fig.10.3 Source: | By KMJ, alpha masking by Edokter - de.wikipedia, original upload 26. Jun 2004 by, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2954889 |
• It then passes through the filament.
• After passing through the filament, the current comes out of the bulb through the other copper conductor.
3. We know that when current passes through a conductor, that conductor gets heated.
• In our present case also, the current is passing through a conductor, which is the filament. So the filament will surely gets heated.
4. But instead of the heat, here we are going to see light.
• This is because, when current passes through the filament, it becomes so hot that, it becomes white in color, and begins to emit heat.
5. But to produce light in this way, there is a condition:
• The filament should be made with a 'suitable material'. Then only it will emit light when current passes through it.
6. Most metals emit red light when they become red hot. But we want warm yellow or white light.
• Also the filament should last for a long time. Then only the bulb can produce light.
7. The great scientist and inventor Thomas Alva Edison tried nearly a thousand different materials before he could successfully make a filament.
■ Today, filaments are made using the metal tungsten.
8. Some precautions are to be taken to ensure the durability of the tungsten filament:
(i) When it is white hot, tungsten will react with the oxygen in the bulb. To avoid this, all the atmospheric air is evacuated from inside the bulb
(ii) Because of the high temperature, tungsten will gradually evaporate. To avoid this, nitrogen is filled into the bulb at a low pressure. Nitrogen is used because it is an 'almost inert' gas and is abundantly available.
9. Filament bulbs are also called incandescent bulbs.
• 'Incandescent' means 'emitting light as a result of being heated'
• We can write: In an incandescent bulb, electrical energy is converted to light energy and heat energy
■ Why don't we make filaments using nichrome?
• Reason 1: Nichrome emits red light when it becomes hot
• Reason 2: Nichrome is not as ductile as tungsten. So nichrome cannot be drawn into thin wires.
♦ We want the filaments to be made of thin wires
♦ These thin wires are made into coiled shapes.
♦ Coiled shape helps to accommodate greater length in lesser space.
■ Light up an incandescent bulb for some time. Turn it off and touch it carefully. We can feel that it is very hot.
• It is clear that a large portion of the electrical energy is lost as heat energy.
• In order to prevent such a heavy loss, many countries have now banned the use of incandescent bulbs
■ However, the heat produced by filament bulbs can be utilised in some special cases. Let us see how it is done:
1. When the filament is heated, we get light.
2. Along with the light, infrared radiations are also produced.
• These infrared radiations can be felt as heat.
3. So the 'spherical glass covering of the bulb' is tinted with red.
• This will reduce the visible light.
• We will thus obtain the heat alone.
• Such bulbs are used in poultry farms to keep the birds warm
Some images can be seen here.
4. The infrared radiations can be concentrated to a specified point by using special reflectors.
• The heat at such points can thus be increased to required values.
• Lamps with such reflectors are used in the field of medical treatment
Now we will see some other methods by which electricity can be used to produce light:
Discharge lamps
1. A Discharge lamp consists of a glass tube sealed at both ends.2. The tube is filled with noble gases like argon, neon, krypton and xenon under low pressure.
3. At each sealed end of the tube, an electrode is fitted. One is the anode and the other is the cathode.
Fig.10.4 below shows an image:
 |
| Fig.10.4 Source: https://commons.wikimedia.org/wiki/File:Glow_discharge_regions.jpg |
• That is., those atoms lose electons and become positive ions.
5. These positive ions (cations) are accelerated towards the cathode.
• During this travel, they collide with the ordinary atoms present in the gas.
6. When this collision occurs, the ordinary atoms are excited to higher energy levels.
• When these 'atoms at higher energy levels' return to their normal energy level, the excess energy is radiated as light.
7. The colour of the light thus produced, depends on the type of gas. It also depends on the pressure at which the gas is filled.
• Neon gives red-orange light • Argon gives violet light • Krypton gives green light • Xenon gives grey light • Sodium vapour gives yellow light
Fluorescent lamps
1. A fluorescent lamp consists of a glass tube sealed at both ends. It is shown in fig.10.6 below: | ||
| Fig.10.6 Source: | By Inkwina (talk · contribs) - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6104284 |
2. The tube is filled with mercury vapour.
3. At each end of the tube, a heating coil is fitted.
4. When the tube is connected to an electric circuit, the heating coils become very hot and they begin to emit electrons.
• Thorium oxide is coated on the heating coils to increase the 'electron emission efficiency'.
5. The rapidly moving electrons collide with the molecules of the mercury vapour.
• As a result of this collision, ultra violet rays are produced.
• But ultra violet rays is not visible light. So it does not serve the purpose of lighting the surroundings.
6. Now, the inside surface of the tube is coated with a fluorescent material.
• The ultra violet rays produced from the mercury vapour is absorbed by the fluorescent material.
7. The atoms of this fluorescent material are affected during this absorption:
• In what way are those atoms affected?
(i) Electrons present in the orbitals of these atoms are excited to higher energy levels.
• Thus the energy level of the atom as whole increases.
(ii) These atoms then try to attain stability by giving off the excess energy.
• For that, the excited electrons fall back to their original orbitals.
(iii) When they fall back, they give off the excess energy. This 'energy which is given off' is in the form of visible light.
• But this fluorescent material needs a 'source'. The ultra violet rays from the mercury vapour is the source
9. The fluorescent lamp requires a very high voltage to get started.
• For that purpose, a ballast is used. The ballast serves another purpose also:
• It prevents excessive current from flowing into the tube
10. The initial cost of fluorescent lamps is greater than that of incandescent lamps.
• But there is a lesser loss of electricity in the form of heat.
• Also they give more light
• So fluorescent lamps are more economical than incandescent bulbs
Compact fluorescent lamps (CFL)
1. The working of a compact fluorescent lamp is similar to that of an ordinary fluorescent lamp that we saw above. Some images can be seen here.2. Both contains mercury vapour.
3. The length of the tube is reduced in a CFL
4. The electronic ballast required for the proper working, is kept at the end of the tube.
• Fluorescent lamps in general contain mercury vapour
• If these lamps are improperly disposed off at the end of their useful periods, the mercury vapour will spread into the atmosphere. This will create health problems
• So we must all work together to create social awareness about this problem
LED lamps
1. The full form of LED is 'light emitting diode'.2. When suitable current is passed through a specially designed circuit, some electrons are able to fall back into lower energy levels. When this happens, they give off energy in the form of light. This is the basis of LED lamps. We will learn more about them when we learn electronic circuits.
3. As there is no filament, there is no loss of energy in the form of heat
4. As there is no mercury vapour, it is not harmful to the environment
5. A small quantity of energy can give a large quantity of light
6. LED lamps can surely help us to fight the energy crisis.
Arc lamps
1. In this type of lamps, two electrodes are fitted inside a glass tube. The glass tube is filled with an inert gas at low pressure. This is shown in fig.10.7 below: |
| Fig.10.7 Source: By Atlant, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=294010 |
3. Then they lose electrons and become ions.
4. When these ions return to their low energy state, the excess energy is radiated as light.
• The colour of the light depends on the type of gas filled in the tube.
5. Here also, the ballast serves two purposes:
• It produces the high voltage required to get started.
• It prevents excessive current from flowing into the tube
6. Arc lamps are used in search lights, movie shooting, flood lights, rescue works during night time etc.,
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