The first diagram shows the Huygens construction for a grating. You can see how the circular diffracted waves from each slit add together in certain directions to give a diffracted wave which has a plane wave front just like the waves hitting the grating from the left. This plane wave is formed by drawing the line that meets all the small circular waves and is called an envelope of all these small secondary waves.
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Diffraction grating maximum: spa
ml = e sinq spa
where m = 0, 1, 2,3...
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The number m is known as the order of the spectrum, that is, a first-order spectrum is formed for m = 1, and so on. If light of a single wavelength, such as that from a laser, is used, then a series of sharp lines occur, one line to each order of the spectrum. With a white light source a series of spectra is formed with the light of the shortest wavelength having the smallest angle of diffraction.