A feather, some hairs and a CD: the story of the diffraction grating
Tilt a CD under a lamp and rainbows slide across it. The disc's tightly packed spiral of data tracks splits white light into its colours, the same trick a Scottish mathematician spotted in a bird feather in the 1670s and that later let scientists measure the wavelengths of starlight.
A diffraction grating is any surface carrying a regular pattern of lines, grooves or slits spaced finely enough to bend light. Each opening behaves like a tiny new source, and the waves from neighbouring openings reinforce one another only in certain directions. Those directions depend on the line spacing, the angle at which light arrives and its wavelength, so different colours leave at different angles. Physicists sum this up in the grating equation, d sin θ = mλ for light striking head-on, where m is a whole number called the diffraction order.
James Gregory, who lived from 1638 to 1675, noticed the coloured patterns a feather produced roughly a year after Isaac Newton's experiments with prisms, making the feather the first known natural grating. Around 1785 David Rittenhouse, an inventor in Philadelphia, built the first artificial one by stretching hairs between two finely threaded screws. Joseph von Fraunhofer made a comparable wire grating in 1821 and, drawing on principles worked out by Thomas Young and Augustin-Jean Fresnel, became the first to record line spectra with such a device and to measure the wavelengths of those lines.
Craftsmanship became the limiting factor. During the 1860s Friedrich Adolph Nobert in Greifswald produced the finest gratings, until Lewis Morris Rutherfurd and William B. Rogers in America took over. By 1900 the concave gratings of Henry Augustus Rowland were considered the best available.
Gratings come as reflective versions, working like mirrors, or transmissive ones, working like lenses, and some alter the phase of light instead of its brightness. Spectrometers and monochromators depend on them, as do optical encoders for precise motion control. The shimmer of oil on a puddle looks similar but comes from thin-film interference rather than any grating.
Source: Diffraction grating
