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Intensity of different color light & its relation to energy AND Amplitude

  1. Oct 5, 2012 #1
    Hi Let's stick to the classical limit.

    We have 2 monochromatic light waves of same intensity. Let's say one is blue and one is red.

    Now this means the individual photons of the blue light have more energy(obviously, higher frequency) and the red light photons have lower energy. But since the two light waves have equal intensity, they have the same energy arriving/second.. Right? I think this part is obvious. This then means that the red light wave has more photons and the blue light wave has fewer photons.

    Well let's move on to amplitude of the light wave, now this sort of scales with the intensity squared so these two light beams should have the same Amplitude? But Amplitude can also be seen as a function of the number of photons..

    Here in lies my current confusion. Sorry if this is so simple. Anyone know of a basic way to get around this and stay consistent?

    Either the two light is going to have different amplitude or the same amplitude while the have the same intensity but different frequencies.
  2. jcsd
  3. Oct 5, 2012 #2
    There is no contradiction. The number of photons in a light wave is dependent on the amplitude. You can calculate the number per second exactly as you described, by looking at the power of the light, and using [tex]E_{photon}=\hbar \nu[/tex] so red light of the same intensity has more photons.
  4. Oct 5, 2012 #3
    Hi, light intensity or energy flux is proportional to photon flux, i.e number of photons per unit time per unit surface and to individual photon energy. Does that answer your question?
  5. Oct 8, 2012 #4
    Hi 0xDEADBEEF if # of photons depends on Amplitude, and if Amplitude comes from Intensity, since these two beams have the same intensity, they will have the same Amplitude, and thus the same # of photons?? (Obviously this isn't the case.)

    "amplitude" is what is throwing me off here.

    Alkim: I understand this but how does this relate with Amplitude?
  6. Oct 8, 2012 #5


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    How do you make that conclusion? If 'amplitude' relates to the amount of energy flux and short wavelength photons have more individual energy then either the same amplitude would imply more red photons or the same number of red photons would imply lower amplitude. (Two ways of looking at the same thing.)
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