# Intensity of a wave in electromagnetism compared to quantum mechanical waves

#### fergal

Hi,
why is it that in electromagnetism we can compute the intensity of a wave by taking the square of the amplitude but we cant do the same for quantum mechanical waves?

My best guess was that its because of the complex numbers in quantum but I'm really unsure.

Any help??

Thanks

#### HallsofIvy

Perhaps you have these backwards. The amplitude of an electromagnetic wave (or any wave) is the intensity- there is no need to square. The difference with "quantum mechanical waves" is that the probability a particle exists at a given point is the square of the amplitude at that point.

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