Can CDs be classified as diffraction gratings.jpg

No, they can not be. This question, perhaps, is prompted from the apparently similar visual colour patterns one observes on the surface of grating and compact disc (CD). Though the mechanism of origin of these colour patterns on CDs and gratings is the same, a CD cannot serve the purpose of a grating.

These colour patterns are due to dispersion of polychromatic white light into individual colours upon diffraction on the non-smooth but arrayed surface of grating or CD. Diffraction is an optical phenomenon associated with the wave character of light that occurs at some edges where the light is incident. For diffraction to happen the size of the edge should be comparable to the size of the wavelength of the light. Gratings and CDs have edges on their arrayed surfaces comparable to the wavelengths (0.40 to 0.80 micron) of visible light and exhibit diffraction and the visual patterns that we observe in common.

But the difference is in the way the arrayed edges are situated on these two surfaces. A grating is a device that has thousands of parallel contiguous groves (and, eventually, parallel contiguous cliffs), called lines (hundreds of lines per millimetre), at the edges of which the light waves undergo diffraction.

A CD, on the other hand, has edges but those of tiny pits (burnt spots) on which the light waves undergo diffraction. Because of the regularity of grooves and hence, edges, the diffracted radiation from grating results in an orderly constructive and destructive interference offering sharp but continuous bands of sequential monochromatic light patches.

Thus one can use the grating as a monochromator (an optical dispersing agent) for many spectroscopic purposes. We come across such orderly colour patches at some characteristic angles repeatedly, known as 1st order, 2nd order, etc with diminishing intensity.

A CD has burnt spots that have low reflectivity (to denote the binary digit ‘0’) at several angular and radial positions (from the centre of the CD) on the surface. The burnt spot has an annular edge with a size comparable to the wavelength of light and does cause diffraction to the incident radiation. The interference pattern occurs randomly as governed by the random distribution of spots and disconnectivity among these spots. The colour patches are irregular, unreliable and feeble. Hence, CDs cannot be placed in the class of gratings.