What is the reason behind diffraction of light?

What is the actual reason behind diffraction of light?

I want to know how is the phenomenon diffraction explained theoretically in physics (best if the discussion limits itself to classical physics).

In certain books, I found that it is usually explained by Huygens principle. But Huygens principle states that each point in the wavefront is a source of wavelets which in turn superimposes to form the wavefront (intuitively speaking), and do not says anything about amplitude of the wavelet neither about the phase. In the Wikipedia article I have found the mathematics version of the principle by frensel, it formalizes the amplitude by a factor $$K(\chi)$$ ,but what does $$\chi$$ means in this context.

So according to Huygens principle light on falling on slit, from each point wavelets are produced and they superimposes but how to know where will the resultant wave go, it may return back (atleast in principle), so which principle completely explains the phenomenon diffraction?

• If you've looked at Fresnel's principle then the way to know the directions(s) is given implicitly in the math: superposition. It's implicit in Huygen's explanation, too, but how evident that is really depends on how Huygen's principle was explained to you. – dmckee Apr 3 at 2:38
• @dmckee Please give the formal statement of the Huygens principle, and instead as far the principle from mathematics standpoint, what is $\chi$? – Bijayan Ray Apr 3 at 8:03