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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.

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It doesn't have a specific frequency -- it has a frequency distribution.

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.

These distributions are called the "wavefunctions" in position and momentum basis respectively, and this duality is the "uncertainty principle" -- read more about this in my quantum mechanics articles here (specifically article 4). In the specific case of white light, white light isn't really a well-defined concept in physics -- it has to do with human eyesight and what visible light entails, but nonetheless the frequency of white light is indeed a distribution with non-zero variance.

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