This is the first step. As light enters from surrounding atmosphere into the polycarbonate layer of the CD, the light bends, or refracts. This is because light travels at different speeds in different mediums. In this case, light travels slower in the polycarbonate layer than in air. The amount of bending depends on the indices of refraction of the two media and can be quantitatively described by Snell’s Law.
Snell’s Law: The law that, for a ray incident on the interface of two media, the sine of the angle of incidence times the index of refraction of the first medium is equal to the sine of the angle of refraction times the index of refraction of the second medium. (Taken from dictionary.com)
θi = angle of incidence
ni = refractive index of the incident medium
θi = angle of refraction
nr = refractive index of the refractive medium
We can take the refractive index of air to be 1, since the actual value is very close to that of vacuum, 1. The refractive index of the polycarbonate layer is approximately 1.58. Therefore, light bends towards the normal when it enters the polycarbonate layer, and bends away from the normal when it re-enters air.
Different wavelengths of light travel at different speeds, so that when a full spectrum of white light passes from the air through the polycarbonate layer of the CD-R, the refraction separates the white light into its constituent colours, which are then reflected back to us by the reflective surface of the CD-R.
The thickness of the different optical mediums, angle of source light, and brightness of source light all affect which rainbow patterns are visible on a CD.
Reflection:
Next, the light rays are reflected off the reflective surface of the CD-R, which is covered with an organic dye. This dye contains pits. The pits will further scatter the light, since the angle of incidence is equal to the angle of reflection. The light that hits the bumps and pits will reflect in different directions as well.
cdsrainbow 