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According to some books, diffraction occurs when there is an obstacle whose linear dimensions are comparable to the wavelength of light. This is true also for a hole or a slit through which the light passes. My question is the following: shall we consider all the linear dimensions of an obstacle (height, width, thickness) compared to the light wavelength? Assuming there is a light beam which passes from a slit comparable with the light wavelength, shall we consider each linear dimension of the slit itself compared to the light wavelength?

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The light is in principle going to be present in a three-dimensional region. In whichever directions the light is restricted, in those same directions it will subsequently spread out, and we call this diffraction. Usually we think of spreading out sideways (the transverse direction) for a beam of light. If we have a pinhole then the light spreads out in two dimensions. If we have a slit then the light only spreads out in the direction restricted by the narrow opening of the slit.

One can also restrict a beam of light in the longitudinal direction by chopping it very rapidly, turning it into a pulse. A short enough pulse will then start to spread out along its direction of travel.

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  • $\begingroup$ Can you explain better if we shall consider all the linear dimensions of an obstacle? $\endgroup$
    – user248666
    Sep 16 '20 at 8:23
  • $\begingroup$ Sometimes some books talk about linear dimensions of a slit but they consider only the width. According to what I have said, it Is important to consider all the linear dimensions. $\endgroup$
    – user248666
    Sep 16 '20 at 10:14
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Actually, diffraction occurs whenever there is a spatial or temporal alteration of the shape of a wave field. However it is easier to observe if it is caused by something with small features or by something that changes very rapidly.

Note that the edge of a large hole or of a razor blade is a very narrow feature, nearly infinitesimally narrow. Light passing an edge is diffracted.

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  • $\begingroup$ How do you explain the expression "linear dimensions" according to lots of book? $\endgroup$
    – user248666
    Sep 16 '20 at 14:15
  • $\begingroup$ The linear dimension of a knife edge in a direction perpendicular to its edge is zero. Many - probably most - authors describing diffraction fixate on the usual: slits, pinholes, knife edges, apertures, and diffraction gratings; and fail to consider the most general cases of diffraction. Andrew Steane's answer, at least, included the temporal version of diffraction, which most authors overlook. $\endgroup$
    – S. McGrew
    Sep 16 '20 at 15:11
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The answer to your question is yes. You can check the Fraunhofer diffraction out of a rectangular slit. You will notice how, according to the dimensions of your slit, you have diffraction over two directions or not.

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