Confused about the appearance of white light fringes

First of all, have I got the right idea in thinking that basically the same effect would appear for both a double slit and a diffraction grating?

I understand that the central fringe would be white, and that blue fringes would be closest to the centre, whereas red fringes would be further out (as they have longer wavelength so it follows that they're diffracted/spread out more)

BUT

why is it that in virtually every white light fringe pattern I've seen, there appears to be three white fringes in the middle - I thought there would only be one central white fringe?

look at this for example: (ignore the pattern for green light above)




also, in my book, it states that 'outer fringes merge into an indistinct background of white light because where fringes merge, different colours reinforce and overlap' - I don't understand how this works, and why it doesn't also occur for the inner fringes

There is an infinite number of 'white fringes' landing on the plane of the viewing screen. Either side of the central and brightest white fringe, there's a repeated pattern of the spectrum of white light. However, as these spectrum is repeated on a particular side, the 'length' of the spectrum increases and the intensity of the maxima/minima decreases also. If that viewing screen were wide enough, it'd just be the same pattern getting longer and less brighter. The white fringes either side are just a very small spectrum of white light you erroneously perceive to be just a 'white light.'

sorry what do you mean by the length of the spectrum increasing? and why does this occur?

so the two 'white fringes' either side of the central one in the diagram, are actually just fringes that contain a bit of white light (along with the other colours)?

whats the two dark lines separating the whiteness in the middle?

consider the main central white fringe. If you look at the range of colours between this fringe and the next, 2nd, white fringe on the right, it's a 'rainbow' spectrum. The 2nd white fringe and the third white fringe (it's small, but it is there, besides the pink) on the right separate the same spectrum spread out over a longer length. This goes on and on and on (in reality, it gets too dark too determine at some point). The stretching of the 'rainbow spectrum' is due to the nature of the waves spreading out as given by the double slit experiment. The fringes also get darker the more you deviate from the central white fringe due to the spreading out of the wave fronts causing a decrease in intensity.