After getting through a course on optics (up to the study of the Michelson interferometer), I'm realizing I have no idea why there is such a thing as diffraction. I can understand why there's refraction and reflection, for instance, and I get there are efficient models for it such as Huyghens', but diffraction seems like magic to me. Could somebody please explain to me the intuitive reason why it exists?
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2Still Huygens-Fresnel – AHusain Dec 23 '16 at 13:15
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Could you elaborate on how it explains the phenomenon? It seems to me it's no more than an efficient model for calculations (whereas for instance in thermodynamics Fourier's law is a model for heat conduction that can be easily explained by invoking the microscopic nature of heat). – Dec 23 '16 at 13:22
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2Possible duplicate: http://physics.stackexchange.com/q/94769/26076 See my answer to this question: diffraction is a kind of interference: resolve a field at a plane interface into a sum of plane waves (by Fourier analysis), propagating in different directions. Owing to these different directions, the plane waves take on different phases in reaching a different plane interface, so they sum to give a different amplitude distribution at other plane interfaces. – Selene Routley Dec 23 '16 at 13:42
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@RaphaelPicovschi The reason behind Refraction..Do you really know?? Well, I don't think that has an answer. We know how it refracts, but do we know why? – Anubhav Goel Dec 23 '16 at 14:01
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@AnubhavGoel Well at least I now have the answer to the first 'why' question! See: https://m.youtube.com/watch?v=Dp4dpeJVDxs – Dec 23 '16 at 14:25
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@RaphaelPicovschi I saw that an year back. That's what my previous comment relayed. I asked you if there is an answer to why (May be you can find). You know how light refracts , reflects or diffracts. But, till date we dont know why. And may be we'll never know. – Anubhav Goel Dec 30 '16 at 16:52
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To understand diffraction in an intuitive way you should take a look at the historical discovery of the phenomenon. The first accurate report and description of the phenomenon was made by Grimaldi in the year 1665.
He performed a simple experiment similar to the penumbra effect. He projected light from an extended source through a pinhole and measured the output. The corpuscular theory of light propagation, which was accepted at the time, predicted that the shadow behind the screen should be well defined with sharp edges. What Grimaldi noticed was that the transition from light to shadow was gradual rather than abrupt. 
The further step was provided by Huygens in 1678. He expressed the intuitive conviction that if each point of a disturbance were considered to be a new sourc of a "secondary" spherical disturbance, then the wavefront at a later instant could be found by constructing the "envelope" of the secondary wavelets. This is the main foundation of the diffraction theory. In essence, the wavelike propagation of light predicts that every point will be a new source of spherical waves.
In 1704, Young strengthened the wave theory of light by introducing the critical concept of interference. The idea states that under proper conditions, light could be added to light and produce darkness.
Then, we have to wait til 1818 when Fresnel combines both theory and states, by making some rather arbitrary assumptions, that the "secondary" wavelets introduced by Huygens might interfere between them. He was able to calculate the distribution of light in diffraction patterns with excellent accuracy. So, in a very simplified way, diffraction is just the "auto" interferences of a wavefront propagating.
Then the rest is just an extend and generalization of that principle through great physicists such as Sommerfeld, Rayleigh, Kirchoff, Maxwell ...
A very good and rigorous mathematical description can be found in the "Introduction to Fourier Optics" from J. W. Goodman.
Marko
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Perhaps consider diffraction as essentially the same effect as refraction at a small scale. How close does the atom or mass actually need to be before it affects the light and you consider the light to be "inside" the medium? The presence of matter near to the light starts to produce the same effect as being inside a medium. Or you could say that in a medium light slows down because it experiences a degree of diffraction. Hope that helps.
JMLCarter
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1Well, it's pretty fundamental, but it is not distinct from refraction it is a different view/model of the same effect, and both models are best used within their respective domain of application. – JMLCarter Dec 23 '16 at 13:42
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Is there any ground behind your philosophy? Refraction and diffraction? How come they are same? – Anubhav Goel Dec 23 '16 at 13:54
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Equations for both can be derrived using Huygens–Fresnel principle. Like the diagrams Marko has in his answer. – JMLCarter Dec 23 '16 at 21:44
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