For this very reason, the difference in optical path between two partial, consecutive beams is quite high: in fact, in the incident direction two corresponding rays coming from two consecutive steps follow the same geometric path; but, while one is following the final stretch, corresponding to height h of the step in the glass, the other follows it in the vacuum and their optical paths differ by (n-1)h (n being the refraction index of the glass).

Since the order m of the spectrum is given by the difference in the optical path divided by the wavelength, in the exemplar that we present here, since h=10.265 mm and making n=1.5 and l=500 nm, the result is m=10,300; moreover, since N=12, the number of the plates, its resolving power is mN=123,600.
In the echelette grating, since the order is quite high, the problem of superimposition is highly accentuated; in fact, to ensure that the wavelength component l¢ of the spectrum of order m is superimposed on the wavelength component l of the spectrum of order m+1, the corresponding differences in optical paths ml¢ and (m+1)l must be equal and the difference l¢-l between the wavelengths of those components must be equal to l/m and for l=500 nm it is equal to 0.05 nm.
To avoid superimposition the incident light must therefore be as monochromatic as possible.
The echelette grating, like all high resolution instruments, is therefore used in the study of the fine structure of the spectral lines: the line to be studied is isolated by means of a prism or grating spectroscope (monochromator) and delivered to the spectroscope slit in which the echelette grating has been placed.