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In the original Michelson-Morley experiment, white light was used and the interferometer produced colored fringes.

Does anyone know why the fringes are colored?

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Because a constant path difference represents a different number wavelengths for the various colours which make up white light. This means that the size of the fringe pattern depends on the wavelength of the light. The coloured fringes are only seen if the path difference is very small so that only fringes close to the zero order are formed. Beyond that the overlap of fringes of different colours is so great that no fringes are visible.

Here are"wedge" fringes (fringes of constant thickness) formed when the mirrors are inclined to one another.

enter image description here.

Note that near the centre of the pattern the red fringes have a larger separation than the blue fringes because the wavelength of red light is greater than that of blue light.

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  • $\begingroup$ Oh, I think I see. I feel silly. When you say "when mirrors are inclined to one another," I assume you're referring to the right half of this image: upload.wikimedia.org/wikipedia/commons/0/01/… $\endgroup$ – Maximal Ideal Jul 27 '17 at 7:15
  • $\begingroup$ @SpiralRain Yes the right hand diagram shows quite nicely the image of one mirror inclined to the other. $\endgroup$ – Farcher Jul 27 '17 at 7:17

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