Diffraction of a laser composed of 2 monochromatic colours through a single slit / $N$ slits To make the example simple, let's say we have a yellow laser that is the combination of 2 monochromatic lasers (green + red). In the example I will give, let's consider that the slits are narrow enough, so that the laser go through all of them.
I came to the understanding that:

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*If the laser go through a single slit: diffraction figure will mostly be a combination of the 2 colours (so yellow, but with different shades since it will be more or less red/green depending on the point we are looking at) and only where it is a zero for one color we will see the other color only.

*If the laser go through N slits (let's say of width and spacing of a few $\mu m$ each): we are going to see the colours, always separated from each other, and only where it is the maximum for them (and nothing anywhere else).


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*Are my statements correct?

*If so, why is it happening like this ? My guess is that we have only diffraction for one slit, and diffraction + interferences for N slits, could you confirm that?

*If so again, does it require a minimum of slits to have a "clean" case like the second one ? What about 2 slits for example?

Note: I am in an entry level course in electromagnetism. So I'm talking about the classic school case, we can set aside particular effects that could actually happen in certain very specific "real cases".
 A: In the case of N transmission slits, you correctly understood that it has a combination of diffraction and interference. There are several different (so-called) orders of diffraction. In the case of $\mathrm{0^{th}}$ order that is just in the same direction as of incident light, you will see the yellow light as there is no dispersion. Those colors will be separated in so-called higher orders of diffraction. As you increase the number of slits the maxima of diffraction order become sharper. The intensity of the peak goes like
$\left(\frac{\sin(Nx)}{\sin(x)}\right)^2$
where N is the number of slits and x is related to the deviation from the peak. If there are only two slits, the peak will be broader, and if you let the diffraction propagate long enough, the color will separate eventually. Yes, it also depends on the width of the slit. Because after passing through the slit, the diffracted light becomes a fan, the width of this fan depends on the wavelength and slit width ($\theta \sim \sin^{-1}\left(\frac{\lambda}{d}\right)$). where $\lambda$ is the wavelength of light and d is the slit width. You can only see interference appreciably if your maxima and minimal lie within this diffraction cone. Hence, the slit width should be small.
