When we shine a flashlight onto a CD, white light decomposes into a rainbow spectrum. This phenomenon is caused by the wave nature of light. Parallel tracks are applied to the disk surface at equal distances from each other. Each track works as a re-emitter. In those directions where the re-emitted waves are in the same phase, they amplify each other when they are added. If you shine a monochromatic laser pointer onto a CD, bright streaks of light are visible in these directions. When we illuminate a disc with a broad-spectrum source, each wavelength has its own spot, and together they give a rainbow spectrum.