Diffraction Grating with a CD
#Presented by Ritik Tiwari and Joydip
Introduction
#Hold a compact disc under a white light and tilt it — the surface shimmers with a shifting rainbow of colours that no paint or dye could produce. The colours are not in the disc itself; they are created by light, and the object responsible is the same microscopic structure that stores your music or data.
In this demonstration, a CD is examined under a light source to reveal the rainbow of colours that appears when the disc is held at different angles. The iridescent pattern is a direct consequence of diffraction: the tightly-spaced spiral track on the CD's surface acts as a reflection diffraction grating, separating white light into its constituent colours the same way a prism does — but through interference rather than refraction. The demonstration connects an everyday object to the fundamental wave nature of light.
Precautions
#Handle the CD by its edges to avoid fingerprints on the reflective surface, which can obscure the diffraction pattern. Do not use a laser or very bright concentrated light source aimed directly at the eyes from the CD's surface.
Materials Required
#- A compact disc (CD) or DVD
- A white light source (ceiling light, torch, or sunlight)
Science Behind It
#A CD stores data as a spiral of microscopic pits and lands pressed into a polycarbonate disc and coated with a reflective aluminium layer. The track pitch — the spacing between adjacent spiral turns — is approximately 1.6 micrometres (µm), which is comparable to the wavelengths of visible light (roughly 0.4–0.7 µm). This spacing turns the CD's surface into a reflection diffraction grating.
When white light strikes the surface, each part of the track reflects light in all directions. Light reflected from adjacent tracks travels different distances before reaching the observer's eye. For a given viewing angle θ, the path difference is d·sin θ, where d is the track spacing. When this path difference equals a whole number of wavelengths (d·sin θ = mλ), the reflected waves from neighbouring tracks reinforce each other constructively — producing a bright reflection. Because different wavelengths (colours) satisfy this condition at different angles, the disc separates white light into a spectrum, and the colour you see depends on the angle at which you hold the disc.
Tilting the disc shifts which wavelength is reinforced at your viewing angle, so the colour changes as you move it — a striking illustration of how the wave properties of light govern everyday appearances. The same principle underlies purpose-built diffraction gratings used in spectroscopes, optical instruments, and scientific instruments that analyse the spectrum of light from distant stars.