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Simple light questions

  1. Dec 3, 2004 #1
    Hey guys!

    I have some very basic questioins about light ..

    What exactly produces light? I know an incadescent light bulb has a filament that is basically a resistor that heats up, but why does it give off light? What is it that makes it put out the light?

    So that leads to the question ... what is light? I know it has dual nature of wave and photons, but does it have mass?

    And where does light go in a room? If you are in a brightly lit room and shut off the lights, where does the light that was in the room go? Is it absorbed by the surroundings?

    I dunno.. Just wonderig :)

    Thank you!
  2. jcsd
  3. Dec 3, 2004 #2


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    Your first and last question can actually be thought of as the same question in two different forms. You are correct in your statement that the filaments in a lightbulb "heats up". The heat in the filament is electromagnetic energy. This energy radiates out from the source in the form of waves (or particles), which somewhat touches on your second question; light is electromagnetic energy radiating out from some source.

    Now, at this point you may be saying to yourself, "but I thought you said heat is electromagnetic energy, and now you're saying that light is". But you see, the only difference between heat and light is the frequency of the waves. Electromagnetic energy with long wavelengths (low-frequency) is the phenomenon we experience as heat. Add a slightly higher frequency, it enters into the range of frequencies we commonly refer to as infrared light, meaning "below red".

    At a slightly higher frequency electromagnetic and he becomes visible to the human eye as red light. Keep increasing the frequency (shortening the wavelength) and you can create all different colors of light that humans can see. The human eye is attuned to see certain frequencies of electromagnetic energy just like the ear can hear certain frequencies of sound. And, just like there are higher frequencies that the human ear cannot hear, there are higher frequencies of light that the human eye can see. The highest frequency the human eye can see is violet, so anything above that is called "ultraviolet", meaning "above violet".

    So, when the filaments in the lightbulb gets enough energy to emit electromagnetic radiation at frequencies high enough to be seen by the human eye, we say that it is "giving off light". When return off the light, the source of energy from the electrical current in the house is discontinued. Most of the light dissipate out into the universe, and gets spread out so thin that we can no longer see it. Some of the is indeed absorbed into the surroundings, but returns to a lower energy state and therefore a lower frequency, leaving the walls of the room slightly warmer than they would be if the light had not been on.
  4. Dec 6, 2004 #3


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    The electric current heats the filament. Since heat is a form of energy, this energy is imparted by the atoms of tungsten in the filament. Electrons in the atom will jump to more external and more energetic orbits.
    Some of the electrons in the energetic orbits will fall again to orbits with less energy. They must get rid of the excess energy and do so emmitting EM radiation: IR, visible light, UV.
    As long as you supply energy to the lamp, the process continues.
    As to what happens to the light, it is absorbed by the walls of the room and by the air. Since energy cannot disapear, the walls and the air are heated and the process of emmitting EM energy is continued, but since the temperature of the walls and air is much lower than that of the filament, the jumping eletrons can only emmit EM radiation of lower frequency, in the IR range, so we cannot see it.
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