I recently learned about the so called "diffraction" grating. In most textbooks I have seen, it is described as multiple slits setup, with really narrow slits. It is sometimes said that the width of the slits is much less than the wavelength of the light going through it. Where is the diffraction in this ? This "diffraction" grating looks much more like an "interference" grating, where the interference come from the multiple slits interfering each others, not from within one slit (diffraction), since those are too small to apply Huygens principle. I should also mention that diffraction gratings are usually being discussed right after the study of diffraction in the textbooks I have checked. This adds to the confusion. So I have been searching on the web to find out what I am misunderstanding with diffraction gratings. And I have come across one PDF file from Harvard (link here) which states (page 13, first paragraph of "Remarks") : "A diffraction grating should more appropriately be called an “interference grating,” because it is simply an example of N -slit interference. It is not an example of diffraction, which we will define and discuss in Section 9.3.1. We’ll see there that a feature of a diffraction pattern is that there are no tall side peaks, whereas these tall side peaks are the whole point of an “interference grating.” However, we’ll still use the term “diffraction grating” here, since this is the generally accepted terminology." So after all, the terminology seems wrong ! Diffractions gratings have nothing to do with diffraction ?! oO However since I don't like that much to rely on one source of information only, I wonder if some people here could confirm this ? Have you also been confused by the bad terminology ? Why is this terminology actually used if there is no diffraction in a diffraction grating ? That sucks.