Here is my understanding:
Non laser light must be passed through a single slit before being directed at double slits. This is to:
1) make the light sufficiently monochromatic such that the double slits emit waves of the same frequency
2) to make the light into a point source such that the double slits emit waves with a constant phase difference
So white light must first be passed through a single slit before being directed at double slits.
If non monochromatic light is directed at the double slits, then the double slits emit light of changing frequencies and so aren't coherent - the interference points constantly change and no pattern is seen.
Therefore I do not understand how white light fringes can form. How do the fringes contain all wavelengths of the spectrum, when we are directing the white light through the single slit in order to make it sufficiently monochromatic. Surely the fringes should contain just one wavelength of light at different intensities instead of all the wavelengths.
Any help greatly appreciated.