"Black" means that light in the visual wavelength range is absorbed (None reflected). However, at the wavelength of the CDP laser the disc must be reflective.
By the way, the laser light striking the CD is always reflected. However, the "pits" in the CD surface are 1/4 wavelength deep so that the light reflected from the bottom of the pit is out of phase with the light reflected from the CD surface, and cancels so that it looks like no light is reflected.
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By the way, the laser light striking the CD is always reflected. However, the "pits" in the CD surface are 1/4 wavelength deep so that the light reflected from the bottom of the pit is out of phase with the light reflected from the CD surface, and cancels so that it looks like no light is reflected.
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