Figure 7.6
Dispersion of radiation by a prism monochromator.frequencies of a recorded spectrum appearing relatively more widely spaced at the high-frequency end
than at the low-frequency end.
Diffraction Grating Dispersion
For most instruments operating in the ultraviolet, visible and infrared regions dispersion of radiation is
produced by a diffraction grating. This consists of a polished piece of glass into the surface of which a
large number of very closely spaced parallel grooves are cut. The surface is coated with aluminium to
render it highly reflective. The grooves must be cut extremely accurately with a spacing that is of the
same order of magnitude as the wavelength of the radiation which is to be dispersed. Holographic
gratings, which are produced by a rapid photographic process, are superior to those which have
mechanically cut grooves. Dispersed radiation of a given wavelength contains less 'stray light', i.e.
radiation of wavelengths other than the nominal one, which leads to a wider range of linear response in
measurements of absorbance as a function of concentration (p. 357 et seq.) The principles of dispersion
by a grating are shown in Figure 7.7.
A beam of monochromatic radiation of wavelength λ falls on a grating with parallel grooves d apart.
Consider two incident rays striking the grating at equivalent points A and B and at an angle φ to the
normal. The rays are diffracted in all directions, one such direction making an angle θ to the normal.
The ray reflected at B travels an additional distance given by
If BD + BE represents an exact number of wavelengths, the rays reflected at A and B will
constructively interfere. At all other values of θ, the net reflected intensity will be zero. Thus, for a
polychromatic beam, the angle of reflection θ will vary with wavelength according to the equation
where n is an integer.
The angular dispersion of the grating is given by
By using the grating so that the value of θ remains within a narrow range