'Diffraction' or 'Interference' gratings ?

  • #1
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.

Answers and Replies

  • #2
To add even more to the confusion, note that the wikipedia page refers to diffraction gratings as having slits wider than the wavelength, thus allowing diffraction to actually occur :

"An idealised grating is considered here which is made up of a set of slits of spacing d, that must be wider than the wavelength of interest to cause diffraction."
  • #3
Ok, I've just read twice on the web that actually, no matter how thin is the slit, diffraction always occur (it just occurs less)...Aha, so all the textbooks I have read are wrong. I guess when the slit width is comparable to the wavelength, diffraction becomes negligible...This is a huge distinction.

Still, it seems that sometimes diffraction gratings exhibit significant diffraction, sometimes not, depending of the slits width.
  • #4
So after all, the terminology seems wrong ! Diffractions gratings have nothing to do with diffraction ?! oO

There's no simple answer for you. Diffraction itself is a case of wave interference. The term is just usually used to describe a wave that encounters an obstacle or different medium, such as a diffraction grating.
  • #5
Natural-language terminology is not always consistent, even in physics. You just have to be aware of the quirks in usage.
  • #6
Ok I see. Thanks guys for the answers!

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