# The Naked Scientists Forum

### Author Topic: Theoretically, could a loud speaker emit light ?  (Read 2490 times)

#### Les the Scientist

• Jr. Member
• Posts: 39
##### Theoretically, could a loud speaker emit light ?
« on: 09/12/2005 07:45:25 »
If the frequency was high enough then surely yes, it could ???
Or maybe it could cook your food by emitting microwaves ?

#### Soul Surfer

• Neilep Level Member
• Posts: 3345
• keep banging the rocks together
##### Re: Theoretically, could a loud speaker emit light ?
« Reply #1 on: 09/12/2005 10:37:49 »
No. loudspeakers are designed generate only air pressure waves by moving materials around and not electromagnetic waves.  Also normal air pressure waves don't make much sense at frequencies above 40Khz because the granularity of the aroms in the air causes them to be highly attenuated.

The coil on a loudspeaker can however generate electromagnetic waves and it does not have to move to do it.  Just putting an alternating current through the coil will do it, but it radiates rather inefficently.  To generate electromagnetic waves efficiently you ned an aeriel or radiating structure that is well matched to the frequency or wavelength that you wish to emit.  FM radio uses 100Mhz waves which are about one metere long so FM aeriels are about half a metre so the sample the maximum difference as the waves move by  TV waves are a higher frequency and so TV dipoles (the bit near the back of a TV aerial are smaller.  Mobile phone aeriels are even smaller because the waves some of them use are only about 10cm long around a couple of GHz.  The waves of light are a bit less than one micrometre so conventional aerials and electrical circuits would be rather small.  That is why semiconductor structures (led's) and quantum mechanical processes  (lasers) are used to generate light waves coherently and efficiently.

OK heating things up a lot creates light and if you put a load of power into your speaker coil it would glow red hot (just before it burnt out!), but that's a different process, you've just got the atoms so excited the emit light by their vibrations a very inefficient way of generating light as anyone who has tried to remove a hot GLS light bulb knows only too well :-).

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#### The Naked Scientists Forum

##### Re: Theoretically, could a loud speaker emit light ?
« Reply #1 on: 09/12/2005 10:37:49 »