In this lecture from MIT, the professor defines the intensity of a wave as the square of the amplitude of the wave. But, at the same time, the professor defines the intensity of light as the number of photons per second. Are these definitions equivalent?

I know that light has both properties of a particle and properties of a wave, but the two different definitions of intensity have different units, which seems confusing to me.

Additionally, when I searched up intensity on Wikipedia, I was unable to find a definition of intensity that matched the definition provided in the lecture. Some of the definitions of intensity that I did find on Wikipedia were "power per unit area," "power per unit solid angle," and "luminous flux per unit solid angle."

Thus, my question is: what do the terms "intensity," "intensity of light," and "intensity of a wave," mean? What are their definitions?

  • 3
    $\begingroup$ This would be much better suited to Physics. $\endgroup$ Jun 27, 2018 at 1:54
  • $\begingroup$ @orthocresol Perhaps, but the MIT lecture was from a chemistry course. Do you think I should post this question in Physics Stack Exchange? If so, is there a way to move the question to Physics? Or do I need to create a separate, but identical question in Physics? $\endgroup$ Jun 27, 2018 at 2:47
  • 1
    $\begingroup$ Why are lasers dangerous to look into? How do they work? $\endgroup$
    – ringo
    Jun 27, 2018 at 4:13
  • $\begingroup$ As each photon has a certain energy, ($E=h\nu$ ) then the power /unit area is the number of photons striking that area per second, so intensity. (btw 'square law detectors' i.e. your eyes , photodiodes etc. detect intensity not amplitude. ) $\endgroup$
    – porphyrin
    Jun 27, 2018 at 6:20
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    $\begingroup$ All those things are the same. You're mixing the wave and particle descriptions of light, so naturally, there are different definitions of intensity depending on the perspective. $\endgroup$
    – Zhe
    Jun 27, 2018 at 12:13


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