After a diffraction problem on our recent, I asked students to explain why the bright fringes even occur. The quotes below included diagrams not shown (which made them even strong), but you can get the gist of student explanations from what they have written. These were definitely the strongest in the bunch.

Strong Examples:

“Bright Fringes are caused, because when the laser light passes through the diffraction grating, it can be treated as a wave. The slit is so narrow that the wave spreads out in a circular pattern toward a screen., and all of this happens in multiple locations on the grating. Along the screen, where the wavelengths of light meet correctly–where they’ve traveled a difference of an integer number of wavelengths (meaning the path difference is a whole number of wavelengths), there is constructive interference. At places where the path difference is  m + 1/2 wavelengths, there are dark spots caused by destructive interference. This destructive / constructive interference can happen in multiple places, so you get a pattern of bright and dark fringes.”

“When light passes through the small slits (they sit side-by-side), the light wave spreads out from each slit in circular patterns. All of this happens right on top of each other so that the waves interference with each. It gets kind of crazy. At places where all the waves land in phase–when one crest occurs will all the other crests for example,  the light waves amplify each other to create a bright fringe. Because there are lots of slits in a diffraction grating, these bright fringes occur more and more infrequently the further you go away from the central bright fringe. They are also very thin–constructive interference only happens in a very precise location. What happens more commonly, on the other hand, is that destructive interference occurs. Because of the many wave fronts overlapping from different slits, they are much more likely to be out of phase, thus creating large sections of dark regions between the bright ones.”

“The bright fringes are caused by constructive interference of the light waves. When the waves are in phase, their crests and trough lines up causing constructive interference. This is a wave property of light. As the waves pass through the slits, they diffract away from grating in spherical wave fronts. When waves from one slit have traveled a certain number of wavelengths, they constructively interfere with waves that came from other slits that have traveled “m” number of wavelengths (m being whole number). When all the waves constructively interfere, the amplitudes of the wave is increased dramatically, making intensely bright fringes.”

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Slightly weaker ones usually had nothing incorrectly said, but may have had idea of constructive / destructive but not clearly linked to path difference and wavelengths. Or a student might have commented on the importance path difference and wavelength, but not linked to the mechanism of constructive interference clearly.

The weakest ones without incorrect information might just have said, “constructive interference”, or mentioned something vague about wave-like behavior with out details.

The incorrect ones almost almost always mentioned either refraction (I think confusing refraction with diffraction/interference) or mentioned something about how only certainly rays make it through the slit (confusing I think with discussion of a pinhole camera, how rays are blocked from entering the camera).

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I didn’t, but I’d like to assess these explanations using something similar to PSET (clarity, completeness, correctness), so that simply saying the word, “constructive interference” would might get less credit than a thoughtful attempt to explain using refraction.