I have a really hard time solving the following problem I accidentally came across today.
Looking at the back of a usual CD one sees coloured bands. This is explained by the fact that the surface (pits and bumps) is effectively a diffraction grating.
But why is this effect already visible when one uses incoherent light, e.g. from a light bulb, whereas the interference pattern of a single or double slit would be missing with such illumination?
[Physics] Why does a CD work as a diffraction grating even with light from a light bulb
diffractionoptics
Best Answer
Coherence is not a yes/no thing. Different sources of light have different coherence lengths. At distances shorter than the coherence length, there are correlated phases. At greater distances, the phases are uncorrelated. In the double slits we use at my school for a student lab, the distance between the slits is 0.6 mm. On a CD, the sizes of the pits and bumps is on the order of 0.001 mm. I think most common sources of light have a coherence length that is between these two numbers, so you get diffraction off of a CD, but not with double slits.