Practical diffraction gratings

The basic requirement of a grating is a set of regularly arranged points which act as scattering centres and re-radiate incident light. The precise details are dictated by the requirements of resolving power and spectral sensitivity. A modest research grating might have 600 slits per mm.

Transmission gratings

In one method closely spaced parallel lines are ruled on the surface of a piece of transparent material such as glass. The maxima for the different component wavelengths will form beams at different angles. Thus the grating separates the colours much the same as a prism, with one important difference. In the grating, longer wavelengths are deflected more than shorter wavelengths, while the opposite is true for the prism. To achieve a sharp separation of colours we must use a very large number of very thin slits close together.

The light passes through glass in this type of grating. Glass absorbs light to an extent which depends on the wavelength and this effectively limits the use of transmission gratings to visible wavelengths.

incoming wave incoming wave d

red light blue light focal plane

Segment of a transmission grating.

red light blue light focal plane

Segment of a transmission grating.

Reflection gratings

The grooves on a compact disc can act as a grating

The grooves are ruled on a highly reflective material such as aluminium sputtered onto a glass surface. When illuminated by the incoming light the edges of the grooves act as coherent light sources in the same way as the slits, but this time the light emerges on the same side as the incoming light. Reflection gratings may be used to examine both visible and ultraviolet light because absorption is negligible. For this reason such gratings are almost always used in spectroscopy research. High quality gratings may have 10,000 grooves per mm.

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