When a beam of parallel light with wavelength λ is vertically projected on the grating plane, the light waves will be diffracted at each slit, and the light waves diffracted through all slits will interfere with each other. Interference fringes formed by diffracted light are located at infinity. If a condenser lens is placed behind the grating, the diffracted light in all directions will converge on its focal plane after passing through the condenser lens, and the interference fringes of the diffracted light obtained are determined by the following formula according to the grating diffraction theory:

(k= 1，2，3，…)( 1)

or

The above formula is called grating equation, in which the distance between two adjacent slits is called grating constant, λ is the wavelength of incident light, K is the series of bright stripes, and K is the diffraction angle of K-level bright stripes. The light interference in the diffraction angle direction is enhanced, and the light interference in other directions is cancelled.

When the incident parallel light is not perpendicular to the grating plane, the grating equation should be written as

(k= 1，2，3，…)(2)

Where i is the included angle between the incident light and the plane normal of the grating. Therefore, it is necessary to ensure that the incident light is incident vertically in the experiment.

If the incident light is not monochromatic light, but contains several different wavelengths, it can be seen from the formula (1) that the central bright stripes of each monochromatic light overlap at the central bright stripes (k=0, =0). Except for the zero-order fringe, for other spectral lines of the same level, monochromatic light has different wavelengths λ and different diffraction angles, so polychromatic incident light will be decomposed into monochromatic light, as shown in figure 1. Therefore, a single chromatographic line arranged in wavelength order will appear on the focal plane of the lens, which is called the diffraction spectrum of the grating. A spectrum composed of spectral lines with the same K value is called K-level spectrum.