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Thread: electromagnetic spectrum question

  1. #1 electromagnetic spectrum question 
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    why is it that light in the infrared spectrum will heat up a surface quicker than light in the visible spectrum even though the infrared spectrum is a longer wavelength with resultantly less photons per unit length?

    I'm waiting for someone to comment that my conception is based on fallacy rather than scientific theory...............!

    is it ultimately to do with the size and sensitivity of the human body that visible light isn't found to be 'hotter' than infrared or am i really off the rails?




    edit - or is it simply a case of how many watts are being transmitted?


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  3. #2  
    AMR showtime's Avatar
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    Visible light have a short wavelength than IR and it always have more “energy per photon”. But there is one thing called intensity of radiation (how many photons radiate per unit length per unit time). IR radiation from the space almost full adsorbing by atmosphere (especially by H2O molecules), but visible light going through atmosphere (with little distortion). Human body (skin) consist 80% of water and IR radiation adsorbing by this water more better than visible light and heat up our skin.
    I hope I understand your question and gave a right answer. :wink:


    Sorry for my english.
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  4. #3 Re: electromagnetic spectrum question 
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    Quote Originally Posted by fatman57
    why is it that light in the infrared spectrum will heat up a surface quicker than light in the visible spectrum even though the infrared spectrum is a longer wavelength with resultantly less photons per unit length?
    What makes you think that ?
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  5. #4  
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    thank you showtime the answer is very much where I was going.

    The reason I asked is IR radiation has a longer wavelength than light & has less photons per unit length. If I heat a pipe to 60 deg.C it feels very hot but does not emit light. Is the heat I am feeling confined to the IR part of the electromagntic spectrum?

    If it is confined to IR wavelengths then light should heat the surface quicker than photons with an IR wavelength, which as far as I am aware it doesn't.

    My intuitive answer would be that 60deg.C is too cool for it to start emitting light, a car exhaust reaches over 1000deg.C & starts to glow red. But then again if I have a light bulb & warm pipe that emit the same amount of energy the light bulb will not give out as much heat.

    As for my definition of heat I am confusing myself - heat can be used very liberally I find to mean energy but is it confined to photons or does it include other matter as well?
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  6. #5  
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    A 60C pipe does not emit light in the visible spectrum because it is not hot enough (Read about black body radiation). An incandescent light operates at a higher temperature and gives off visible light. How do you know that visible light will not heat a surface faster than IR? To compare the two you would have to measure the intensity of each, which you probably don't have any way of doing. The photons are not heat. Heat is a property that matter has due to a temperature change or phase change.
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  7. #6  
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    so what is the technical definition of heat?

    wiki doesn't seem to help here................says heat is energy transfer which I kinda knew already but I am looking for a more precise definition if it is available.



    found this link which look like it comes from caltech:

    http://coolcosmos.ipac.caltech.edu/c...rmal/heat.html

    Is this deemed correct by forum members?
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