# What does “intensity of light” mean? [closed]

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?

## closed as off-topic by jonsca♦Sep 27 '18 at 23:13

• This question does not appear to be about chemistry within the scope defined in the help center.
If this question can be reworded to fit the rules in the help center, please edit the question.

• This would be much better suited to Physics. – orthocresol Jun 27 '18 at 1:54
• @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? – user62238 Jun 27 '18 at 2:47
• Why are lasers dangerous to look into? How do they work? – ringo Jun 27 '18 at 4:13
• 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. ) – porphyrin Jun 27 '18 at 6:20
• 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. – Zhe Jun 27 '18 at 12:13