Structure of white light? White light is a mixture of different wavelengths. 
If so what will be the structure of a beam of white light ? Is there a separation between different colours ? what does it actually mean ?
Does a beam of white light shows any spacing between different wavelengths ?

 A: The general idea is that if you had empty space, with only visible white light moving through it, and you put a line of electrons a few thousand nanometers long, they would start accelerating up and down, maybe in a pattern like this:

where each tick of the horizontal axis is something like hundreds of nanometers and the vertical axis is small. 
This looks like it has no rhyme or reason to it, but it is actually the sum of a lot of different things. I take the function for red light (say it's of the shape $\cos(x+\varphi_1)$ and it propagates to the right at the speed of light), and I add it to the functions of higher and higher frequencies ($\cos(2x+\varphi_2)$ and $\cos(3 x+\varphi_3)$ and so on). The variable $\varphi$ are just arbitrary constants. 
In an idealized detector, even though you've summed them, an individual frequency will activate an individual sensor. It doesn't matter what else you add into the function - if it's not precisely the frequency you want, the sensor won't get activated.
So, when all of your sensors are being activated, you can be sure you're looking at a sum of these ideal frequencies. And that's what white light is like.
(I'm trying very hard here to NOT write about Fourier transforms, and ignore difficulties with human rods and cones, and ignore the difficulties of "pure frequencies" when the source of light has finite length in time, and also to ignore the difficulty of what the spectrum of white light "actually" looks like and accurately reproduce how much the frequencies taper off!)
(The graphic was generated with the mathematica code Plot[Sum[Cos[n t + RandomReal[2 Pi]], {n, 1, 40}], {t, 0, 2 Pi}, Evaluated -> True])
A: White light is composed of photons of varying energies. The photons themselves do not have to have any perpendicular spatial separation as your picture suggests. Rather, it is possible that the photons of different energies are coming along a single path, one after another.
