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Thread: a question about light

  1. #1
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    a question about light

    this is probably a stupid question, but i've been wondering. i know that in order to radiate visible light, something must be very hot (like the surface of the sun). but why does my light bulb radiate the same visible light when it is so cool?

  2. #2
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    Something doesn't have to be hot to radiate heat. Heat and light are both forms of radiation. An object can give off one without giving off the other. Or it can do both. It all depends on the substance and circumstances.

    For example, your lightbulb gives off both visible light and heat, but a chemical glowstick only gives off light. The chemical reation in the stick realeases light, but very little heat (if any, I'm not sure about the specifics)

    Other radiation (such as ultraviolet) may also be released. This is how tanning beds work.

    Anyway, I hope that's helped.

  3. #3
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    Hi, heat and light are different types of energy given off by the sun but both are a bi-product of the fusion of hydrogen nuclei at the centre of the sun to form helium nuclei. This fusion occurs because of the immense pressures and heat at the centre of the Sun which is approximately 15 million degrees kelvin! It is a lot cooler on the actual surface at approximately 6000 kelvin. Scientists on earth are trying to replicate the nucleur reaction that takes place in our Sun because it produces huge amounts of energy with little or no radioactive waste i.e. its a much cleaner and efficient way of producing energy.

  4. #4
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    ... and a light bulb gives off heat and light due to current passing through a resistor. The amazing thing, though, is that both the sun and a lightbulb filament can be thought of as approximate black bodies. A 100W light bulb has a colour temperature somewhere around 3000K, roughly half that of the sun. That's really hot! So, why don't we burn up?

    A black body emits at all wavelengths. Its colour, though, is determined by what wavelength it emits at the most. A 100W lightbulb at T~3000K emits most of its light at ~960nm, which is in the IR part of the EM spectrum. That's not the temperature you feel, though, as a lot of the energy given off in the IR band is absorbed by the glass of the lightbulb itself, and the air surrounding it. We see light from the light bulb because, though the peak of the Planck curve is in the IR, it has a tail in the visible, which we can see with our eyes. It has a negative slope in this region, meaning that the light bulb gives off more red light than blue light. The sum of all the light given makes the light appear kind of yellowish.

    This is different from, say, a UV lamp which works by exciting electrons in an inert gas inside the tube. As the electrons fall back down into lower energy state, light of only a finite number of wavelenghts is given off, most of which are in the visible band. This is why UV lamps are much cooler than standard lightbulbs.

    So, uh, yeah... Light bulbs are hot. Really hot. It's best not to touch them.

  5. #5
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    A flourescent light is just an UV light with a phosphorescent coating on the inside that emits visible light when it is struck by UV light.

  6. #6
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    Quote Originally Posted by The Supreme Canuck
    Heat and light are both forms of radiation.
    Quote Originally Posted by Mars_Landing
    heat and light are different types of energy given off
    These statements strongly imply that there is some fundamental difference, like the erroneous statement that "heat is infrared radiation," which is untrue.

    Hot things radiate according to something close to the blackbody spectrum, and that has a lot of energy in the infrared for things we consider "hot." But heat is just energy being transferred, and there are ways of doing that via radiation without a blackbody. Otherwise microwave ovens wouldn't work.

    To say e.g. a lightstick gives off light without getting hot is not the same as saying it doesn't give off heat. I've burned myself with less than a watt of visible light.

  7. #7
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    You will also note that the temperature of an ordinary household lightbulb filament is the same as the temperature of many red dwarf stars;
    if you were to go to Proxima Centauri, say, or Gliese 876 you would see everything illuminated by a light the same colour as the light in your house at night.

    Red Dwarfs are only red by comparison with the sun; in fact you would have difficulty seeing any difference, if you went to visit one.

  8. #8
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    Quote Originally Posted by swansont
    These statements strongly imply that there is some fundamental difference, like the erroneous statement that "heat is infrared radiation," which is untrue.
    True. I could have worded that better.

  9. #9
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    a follow up question: how hot are ordinary 100 watt light bulbs?

  10. #10
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    Which part? And define temperature. The glass? The filament? And do you mean air temperature? Because there's not really any air inside a light bult.

    The filament radiates at between 2000K and 3000K. Add 300 for C. Roughly double it for F. The glass is far less hot. Maybe around 500K.

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