Light reflection on CD

Explain the colours that are seen when light from a lamp is reflected on a CD.

Light is made up of a collection of many colors. A compact disc is made of a polycarbonate wafer which is coated with a metallic film, usually an aluminum alloy and film is covered by a plastic polycarbonate coating. The coatings are very thin less than 100nm thick and each coating partially reflects and also partially transmits incident light. Light is made of waves-like the waves in the ocean. When light waves reflect off the ridges on CD, they overlap and interfere with each other. Sometimes the waves add together, making certain colours brighter, and sometimes they cancel each other, taking certain colours away.

When light from a lamp is reflected on a CD light rays reflected from different coating boundaries interfere with each other to produce the colorful patterns. The reflectance of the CD is not uniform, because CD disk contains a long string of pits written helically on the disk. Like water drops in falling rain, the CD separates white light into all the colours that make it up. The colours we see reflecting from a CD are interference colour, like the shifting colours on a soap bubble or an oil slick. The reflected light will make rainbow colours: red, orange, yellow, green, blue, indigo, violet. If the coatings thickness is very thin, then interference can occur. Violet light has a wavelength of about 4,000 angstroms (hundred-millionths of a centimeter), while red light has a wavelength of about 7,600 angstroms