# What is the frequency of white light? [duplicate]

We can see white LED light after a combination of red, green & blue LED together. Then what is the frequency of white light? Whether it lies between 380 Hz - 700 Hz?

There has to be a frequency as every physical object carry its vibrational frequency. Or we have to conclude by saying that there is no existing physical colour called white. It is energy, not a colour. That means we see a direct form of energy through our eye.

You don't even need to go as far as white light -- just consider a "camel hump" wave, like $$\sin ax+\sin bx$$ -- what's the frequency of a light wave that looks like this? The answer is that its frequency isn't a fixed value, but a distribution, taking values $$a/2\pi$$ and $$b/2\pi$$ with half probability each. In general, if you have some function $$f(x)$$, the way to obtain this frequency distribution is to decompose $$f(x)$$ in terms of sinusoids -- this is precisely the Fourier transform.
In the specific case you mentioned, position and momentum ("frequency") are "Fourier duals" of each other. If you have a sinusoid (by which I mean $$e^{2\pi i\xi x}$$), you have complete uncertainty about the position, but have a precise value for the momentum: $$h\xi$$. On the other hand, if you had localised your position completely (to a Dirac delta function), you would find a sinusoid in momentum-space.