Cause of diffraction

Phenomenologically, diffraction refers to the propagation of waves when they encounter obstacles or small holes, but the essence of diffraction is actually an interference. We use the principle of Fresnel-Huygens wavelet to deal with diffraction quantitatively. "Wavelet" refers to any point on the wave front of a series of waves (for example, the wave front of a plane wave is usually a plane with a finite size, and the wave front of a spherical wave is the sphere that the wave first reaches at that moment), which can be regarded as a new wave source that emits spherical waves forward, and the infinite small spherical waves emitted by these infinite new wave sources are the wavelets of the original series of waves. The envelope surfaces of these wavelets (that is, the surfaces tangent to the wavefronts of all these wavelets) are the new wavefronts of the original wave series. With the help of the concept of wavelet, we can understand that single-slit diffraction is essentially the interference between different wavelets on a slender strip surface exposed at a single slit. Surface integral is needed for specific processing. Only by learning mathematics well can we fully understand diffraction. For a further description of diffraction, let's look at the first half of my answer to another question-