How does destructive interference occur on a CD grooves/ridges?

If the laser light being shone onto the CD has a wavelength of 500nm and the height of the ridges/grooves is 125nm, how does destructive interference happen?





I know that the waves have to be out of phase when they reach the ridge so that an amplitude is created, which is destructive interference, but how does it get to be destructive? How do the ridges cause the light to be out of phase?





Any help at all would be greatly appreciated. Many Thanks!!How does destructive interference occur on a CD grooves/ridges?
I will say two possible solutions .





First solution.





CD surfaces are not smooth , as you said they are ridges/grooves. Light beams passing through these can be refracted and may have a path difference or phase angle 180. So destructive interference can take place.





Second solution


Light that is reflected form the surface is also inverted . The reflected wave can have different phase angles and so destructive interference may take place and also standing waves are formed .

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