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Wikipedia defines diffraction as -

Diffraction is the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture.

So, we can say that the bending of any light waves around any object is diffraction.

Now, I was reading the book Introduction to Solid State Physics by Charles Kittel and I came across the following line on page 37, Chapter 2 of the fifth edition of the book.

The diffraction depends on the crystal structure and on the wavelength. At optical wavelengths such as 5000Å the superposition of the waves scattered elastically by the individual atoms of a crystal results in ordinary optical refraction.

So in this way it appears that the book is hinting that refraction is simply a special case of diffraction in which the wavelength of light is much larger than the interatomic spacing.

Is this true or have I completely misunderstood the line in the book?

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  • $\begingroup$ That's not really what it's saying. It's saying that the very distinct phenomena of diffraction and refraction both result from the same underlying physics (each atom emitting waves in response to the incoming wave). $\endgroup$
    – knzhou
    Commented Apr 5 at 19:52
  • $\begingroup$ So the same phenomenon causes diffraction and refraction depending upon the wavelength of light used. Just like EM force is the cause behind both very distinct phenomenon of friction and van der waals force. $\endgroup$ Commented Apr 6 at 3:45
  • $\begingroup$ If this is true then what is the name of this underlying phenomenon? And where can I read more on these topics? $\endgroup$ Commented Apr 6 at 3:51

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Diffraction is caused by the confinement of the lateral extent of a wave, and it is relevant, if light is confined to a sizes comparable to its wavelength. The term diffraction has been defined by Sommerfeld as "any deviation of light rays from rectilinear paths which cannot be interpreted as reflection or refraction." Thus, mixing these to terms is not fruitful -- in my perspective.

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  • $\begingroup$ Ok thank you. So diffraction is seperate phenomenon from refraction. $\endgroup$ Commented Apr 6 at 3:43
  • $\begingroup$ Where can I read more on these topics? $\endgroup$ Commented Apr 6 at 3:52
  • $\begingroup$ I don't know. I read it in "Fourier optics" from Goodman, but in this book it is just a side remark. Maybe start with standard textbooks on optics, such as the one from Hecht, Haken and Wolf, or Born and Wolf. $\endgroup$
    – Semoi
    Commented Apr 6 at 10:34
  • $\begingroup$ Ok thank you. I'll look into them. $\endgroup$ Commented Apr 6 at 18:38
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So in this way it appears that the book is hinting that refraction is simply a special case of diffraction in which the wavelength of light is much larger than the interatomic spacing.

Is this true or have I completely misunderstood the line in the book?

He is talking about wavelets/Huygen's principle/the Huygen-Fresnels principle. This principle says that every point of a wavefront can be considered to be a source of waves (wavelets).

This principle underlies the optics class derivations of diffraction through slits. It can also explain cases of refraction. So, it can be considered the foundation of diffraction and refraction in some limits of applicability.

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