1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Regarding heat property of electromagnetic waves

  1. Sep 25, 2009 #1
    I am confused regarding the heat properties of electromagnetic radiation. Wiki states "Any electromagnetic radiation can heat a material when it is absorbed.". Does this imply radiation with wavelengths in the visible light region can also heat a material, that is, increase its temperature? From wiki again: "heat is the process of energy transfer from one body or system due to thermal contact, which in turn is defined as an energy transfer to a body in any ". As light is energy, this should imply that light is heat?

    If that is the case, why does purely visible light without any additional radiation not raise the temperature of the object it shines on? Or does it, so small that it isn't noticable without measuring instruments?
  2. jcsd
  3. Sep 25, 2009 #2
    Energy is energy. Visible light DOES raise carry energy. Have you never stood outside in the sunlight? It is detectible by your brain, without instruments. Sun shines on your body, you get warm. In a sense the statement that light=heat is true. Both are forms of pure energy.
  4. Sep 26, 2009 #3
    I figured that since the sun emits a large portion in the IR region that it was the IR that caused the heating, since something that emits just light like say, a flashlight, doesn't have the same effect like standing outside. Not even a really big flashlight. Guess it's just a difference in magnitude then. Thank you.
  5. Sep 26, 2009 #4
    As far as the molecular absorbion spectra of your skin I don't know. Can someone else speak to this, and is the absorbtion caused by the energy levels of molecular rotational modes or random translational motion both or something else? Also I'm guessing that a flashlight emits a fair amount of its radiation in the infrared range, its going to emit approximately as a blackbody.
  6. Sep 26, 2009 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    It's a misnomer to say that infrared radiation (or any EM radiation) is the same as heat. Heat energy as due to the random motion of a collection of molecules, atoms, or other collection of masses.

    Objects which possess heat energy (i.e. anything with a temperature above 0 K, i.e. anything period) convert some of the heat into electromagnetic radiation. The radiation may then be absorbed by another object and converted to heat energy in the second object ... so EM radiation is a form of transport for heat energy. But that's different than saying it is heat energy.


    Yes. Any standard filament lamp emits a higher fraction of it's radiation in the IR than does the sun, with a peak emission wavelength of about 1.0 μm. Compare that to the sun's peak emission at around 500-550 nm.
    Last edited: Sep 26, 2009
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook