Why do we see rainbow colours on a CD?

Display of colours by a CD can be understood in terms of the working of a plane optical reflection grating. It is a flat optical device whose surface is ruled (striped) with a set of closely and uniformly spaced lines, such that light is reflected by the gaps and absorbed by the lines. , When light falls on a plane reflection grating, it is scattered in all directions by each of its reflecting stripes. These waves from individual gaps are termed wavelets. When we look at the grating from a distance, wavelets from different stripes travel different distances to reach the retina of our eye. Their crests (or troughs) do not reach a given point at the same time. Usually crests of some wavelets and troughs of others reach a point. Troughs have the property of partially or totally nullifying crests and vice versa, depending on their strengths. In this case the wavelets are said to interfere destructively.

But for certain orientations of the grating, it so happens that troughs (or crests) of all wavelets reach a point together, enhancing the effect of each other. The wavelets are then said to interfere constructively. Ordinarily, when light falls on a grating, wavelets corresponding to all wavelengths (or colours) are sent out by the reflecting gaps. Since the conditions of constructive interference hold good only for some particular wavelength, light intensity at the receiving point is exceptionally high only for that wavelength. Light from the grating from the related direction is thus rich in the corresponding colour. Similarly light from slightly different direction is rich in another colour. A CD has a data recording track, which spirals from its outer periphery to the inner circular boundary. This takes many tens of thousand rounds about the CD's centre. When examined along a radius of the CD, it is found to have a structure similar to that of a reflection grating — a set of almost straight tracks running perpendicular to the radius and separated by gaps. Therefore, like a grating the CD also displays colours. Courtesy : The Hindu