Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why we cannot see light from a conducting wire?

  1. Aug 28, 2014 #1
    When I connect filament(light bulb) in the electric circuit I can see the light.

    but when I connect only conducting wire(copper line) in the electric circuit I couldn't see the light from the conducting wire even though conducting wire less resistant than filament.

    How can I explain that phenomenon?
     
  2. jcsd
  3. Aug 28, 2014 #2

    Dale

    Staff: Mentor

    The light emitted from the wire is too low of a frequency for your eyes to detect.
     
  4. Aug 28, 2014 #3
    Light is an electromagnetic wave but it has very big frequency ( frequency of some Terraherz).

    The electric current that runs through the filament of the light bulb or the filament of the copper wire doesnt have that big frequency so normally if EM-waves were created only by current distributions, then we wouldnt have light from either case.

    Then how light is created by the filament of the light bulb? It is because the free electrons that are the electric current collide with the orbiting electrons of the atoms of the fillament , and those orbiting electrons gain energy and give back some of their energy as em-waves with frequency of some Terraherz. The same happens with copper atoms but the orbiting electrons of the copper atoms cant give back em-waves in the frequencies of Terraherz. That is it depends on what exactly atoms we have as to if their orbiting electrons can produce em-waves in the frequencies of some Teraherz.
     
  5. Aug 28, 2014 #4

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The filament in a lamp is a "conducting metal". It is Tungsten, chosen because it can run at a very high temperature ('white hot') without vaporising. At such temperatures, the EM radiation is visible. A length of thin copper wire will glow brightly when it's connected across a beefy battery but it will melt and vaporise very quickly. If you try this, you have to be very careful handling the wire or you will burn a groove in your fingers. It's the sort of daft thing that I did as a boy with an old battery of my Dad's - and I burned my fingers!!!! Beware.
    There is a pretty graph on this link that shows the relationship between temperature and emitted spectrum of a hot body. The spectrum is not much affected by the material used.
     
  6. Aug 28, 2014 #5
    We need resistance to increase the temperature which in turn causes photon emission. But increased resistance also lowers current, so brighter light-bulbs actually have lower resistance, I think. I'm not sure how that works out, I guess it has to do with the filament thickness. But with constant current higher resistance should produce more light, or explosion even?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook