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Why is it that a lightbulb requires a filament, or a different process, (depending on the type of lightbulb) while a radio transmitter uses an antennae? They are both photons, no?
Question Date: 2011-02-05
Answer 1:

A light bulb and an antenna both emit electromagnetic energy--you are right on that point. How and why they do it are very different, and the kind of electromagnetic energy they emit is very different.

In an incandescent light bulb (fluorescent and LED bulbs work differently), an electric current is pushed through a filament (wire), and the filament gets so hot that it glows. (It gets hot because of high resistance, which is like friction to electrons. So it gets hot for about the same reason your hands get hot when you rub them together hard.) Incandescent bulbs waste a lot of energy, because they are putting out a lot of heat and electromagnetic energy other than visible light, including radio waves.

Antennas, on the other hand, emit or receive very specific wavelengths of electromagnetic radiation, and this calls very different physics. (As a side note, the longer the antenna, the longer the electromagnetic wave it is tuned for, just like a longer guitar string plays a lower note.) To send a signal using an antenna, a current is run through the antenna, but--different from a light bulb--the electricity needs to be at a specific frequency; that is to say, the electrons need to change direction very often, thousands to trillions of times per second. Also, there is very little resistance in an antenna, and it heats up only by accident.

So, in a light bulb, a high-resistance wire is being used as a heat source that gives off light, and in an antenna, a specially shaped wire with a very specific pattern of electricity creates electromagnetic waves. The shape and material seems similar (wire and thin metal), and how you activate them seems similar (running electricity through them), but the details make them very different.


Answer 2:

The reason why you use an antenna for radio waves and a filament for light is the relative wave-length of the two waves. You arec orrect in that they are both electro-magnetic waves, however radio typically has a frequency between 100kHz and 100GHz so has wave lengths between 3km and 3mm. Light in the green is about 500nm or 0.0005mmwavelength. You can indeed radiate light with an antenna if you can make it 250nm long -- but this has only recently become possible with nanoscopic fabrication. Normally, light emission is restricted to chemical processes on the molecular scale such as flame oxidation or molecular vibration such as heat. In a filament, a current creates huge amounts of electron scattering leading to large amounts of vibration in the material, on scales from the molecular level up -- thus a glowing filament produces light -- and lots of infrared light with wave lenghts up to 50um or so. This is called "black-body" emission and was the only way to make artificial light for a long time. (The sun is also ab lack-body emitter -- but at a temperature of 5700K). More recently,chemical and material emitters (i.e. florescent and LED) have been developed that are much more efficient in that the ratio of light to heat is far better than is possible in a metal based filament. (About90-98% of the filaments energy is emitted in the infrared).

For further information on Black-Body radiation and light wavelength antennas, there are wikipedia articles. LED lighting is a specialty of UCSB research and you can find several articles on-line from the lighting institute. Piezo resistivity has several applications (the spark in gas lighters is produced by a crystal that is thumped to build a charge), and there are science experiment kits sold to experiment with it.

Nice questions!

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