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Why is the laser diffraction pattern from a line curved when the laser approaches the line with oblique incidence?enter image description here

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  • $\begingroup$ Please crop and shrink the huge photo. $\endgroup$ – user4552 Dec 13 '19 at 21:03
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You can think of the hair as being cylindrical. In your imagination, scale the hair-sized cylinder up to, say, a half inch diameter. Illuminate the cylinder with a beam as wide as the cylinder, at the 20 degree angle of your drawing. Now consider all the directions the light will be reflected into at different points of incidence on the cylinder. You will see that the reflected rays will be confined to a conical range of angles corresponding to the curve you've drawn.

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  • $\begingroup$ Would it still be a diffraction pattern or just a reflection pattern. According to diffraction theory each point acts like a new source and sends spherical wave equally in all directions, not just the reflective angle. Then the two waves from either side of the cylinder interfere constructively or destructively producing the diffraction pattern. $\endgroup$ – Swap Dec 15 '19 at 1:31
  • $\begingroup$ In the scaled-up scenario I described, if the cylinder is silvered, the resulting pattern will be almost entirely due to reflection. The curved pattern you see when bouncing light off a hair is due to reflection, even if some diffraction occurs. $\endgroup$ – S. McGrew Dec 15 '19 at 4:05

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