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B Can the temperature of an object be derived from the EM Wave

  1. May 10, 2016 #1
    Can the temperature of an object be derived from the EM Waves it emits?

    I know that everything having a temperature over absolute zero emits thermal radiations. The hotter the object gets, the higher the frequency of the wave goes.

    But for example if I have a hammer which emits a wave xHz, will I be able to calculate it's temperature by merely studying the waves emitted? (something to do with energy?)

    Please help I feel clueless.
  2. jcsd
  3. May 10, 2016 #2


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    You can indeed measure the temperature by measuring the emitted spectrum of EM radiation from the object. Note that all objects emit a broad range of frequencies, with the peak frequency (frequency with the largest proportion of energy) increasing as the temperature of the object increases. Here's a calculator you can play around with to see how the emitted wavelength and frequency reacts to changes in temperature.

    You'd also need something capable of measuring the radiation. For a room temperature object, the emitted radiation falls mostly in the long-wavelength infrared region, so you'd need a sensor capable of seeing that.
  4. May 10, 2016 #3

    Practically, infrared thermometer already has been used with a principle of the blackbody radiation. Maybe you know that in a hospital we can easily see that thermometer. Maybe you know that Stefan-Boltzmann law says the power of radiation per unit surface.

    Thank you.

    (if you have questions more, send me email to [e-mail address removed by moderator], and if you are a native speaker and rectify my English error, I would really appreciate it because I am learning English.)
  5. May 10, 2016 #4


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    It's the only possible way of determining the surface temperature of the Stars we see. You assume that the emitter is 'black body' and the emission spectrum will tell you the temperature. Red hot and white hot?
    Last edited: May 10, 2016
  6. May 10, 2016 #5


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    Staff: Mentor

    You need some additional information about the object to do that, otherwise you can just calculate a minimal temperature. The problem: not every object is a blackbody. Most objects are good approximations to that in the infrared range, so infrared thermometers work nicely, but some objects emit significantly less radiation than a black body would do, and their emittance ratio relative to a blackbody can have a shape that mimics a body at a lower temperature.

    If you put a warm metal in a cold room and try to measure its temperature with an infrared thermometer, you'll underestimate its temperature, because the metal won't emit much radiation on its own and the power of reflected infrared radiation from the room is low as well.
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