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Echelon Grating

An echelon transmission grating consists of a number of optically worked glass plates arranged in the form of steps as shown in fig. All the plates are cut from a single optically worked glass plate. Each plate overlaps on the next by the same distance i.e. the step width is the same throughout and is of the order of 1mm. A parallel beam of monochromatic light incident normally is diffracted through a small angle.

One can observe the fifth or sixth order spectrum with a concave grating and only the second or third order spectrum with a plane diffraction grating. The resolving power of a ruled grating is dependent on the number of lines on the grating surface and the order of the spectrum. It is difficult and very tedious to draw a large number of equidistant parallel lines on a grating surface. With an echelon grating, designed by Michelson, one can observe the spectrum of a very high order and hence the resolving power of the echelon grating is very high. But, with the increase in the order of the spectrum, the intensity of the spectral lines decreases considerably. Also, the angular spacing dθ is very small and hence there will be many overlapping orders. An echelon grating, therefore, is not suitable to study the spectrum as such but essentially helps in detecting the true monochromatic of a beam of light.