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 Theme 2: Energy and the Environment Background Material Sources of Light
History
What makes a light bulb give off light?
The simple answer for "what makes a light bulb give off light" is that electricity flowing through a wire filament, in an incandescent light bulb, heats the wire filament white hot. The filament of an incandescent light bulb is simply a resistor - it resists the flow of electrons. The filament's temperature rises until it gets rid of heat at the same rate that heat is generated in the filament. The filament's temperature is very hot (2550 °C). At high temperatures, the thermal radiation from the filament includes a significant amount of visible light. An analogy of a log burning in a fireplace might help explain what happens in the filament. The fire-light is produced by the burning wood or heating of the byproducts of the combustion (gases). When any object is heated its atomic particles are moving more rapidly than when this object is cooler. As the wood atoms are heated, some transfer their motion to other material, release energy and then cool down. The released energy of the atoms is in the form of electromagnetic radiation and, if it reaches a certain level, is visible to us as light. The tungsten wire filament in the light bulb is also heated to a high temperature and so gives off electromagnetic radiation. The heat comes from the collision of electrons with the metal atoms found inside the filament wire. The wire would burn if it were not in the glass bulb from which the air has been pumped out and replaced with an inert gas. Even the tungsten wire with its very high melting temperature will melt and the bulb would "burn out" if the temperature gets too high. If the temperature is too low, the light bulb gives off mostly heat and little light. Light bulbs are designed to operate from a given voltage range so that they last a reasonable amount of time and give you white light. If you put your hand close to the light bulb you will feel the low frequency electromagnetic radiation as heat. Most of the radiation from an incandescent light bulb is heat and not light. A Light Emitting Diode (LED) is a special diode that emits light when connected in an electrical circuit. A clear or coloured epoxy case surrounds and encloses a semi-conductor chip.
The two wires extending below the epoxy enclosure, or the "bulb" indicate how the LED should be connected into a circuit. The negative side of an LED is indicated in two ways: The negative lead from the LED should be connected to the negative terminal of a battery. The positive lead from the LED should be connected to the positive terminal of the battery. LED's require very low voltages (1 to 4 volts) and draw a very small current flow (10 to 40 milliamperes). Applying voltages above this range, without adding the correct amount of resistance in the circuit, will result in melting the LED chip. The critical component of the LED is the semi-conductor chip located in the centre of the bulb. The chip has two regions separated by a small space or junction. The p region of the semiconductor chip is mainly composed of positive electric charges. The n region is dominated by negative electric charges. The junction acts as a barrier to the flow of electrons between the p and n regions. If the LED is connected to a power source (battery or generator) with a large enough electric potential difference (voltage) current can flow across the junction from the n region to the p region.
LED and Light Each time an electron recombines with a positive charge, electrical potential energy is converted into electromagnetic energy. For each recombination of a negative and a positive charge, a quantum of electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the semi-conductor material used. Only photons in a very narrow frequency range can be emitted by any material. LED's that emit different colours are made of different semi-conductor materials, and require different electrical potential differences (voltage) to cause the flow of electrons.
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