Colored Bands in Thin Films

The colored bands in soap bubbled or oil films are due to the constructive interference between light reflected from the top and the bottom surfaces of the thin films.

An ink layer with thickness t is coated on a CD surface. The path difference ( p ) between the ray that reflects from the air-ink surface and the ray that reflects from the ink-disk surface is p = 2 t / cosqr ( qr is the refraction angle ). The resultant waves interfere constructively if

path difference ( p ) equals l, 2l, 3l ...

For light viewed at a slightly larger angle, the path difference will be larger. Another color of light with longer wavelength which under wave constructive interference will be seen next to it.