Question

According to Huygens's principle, what is the explanation for boundary behaviors?

Answer 1

The Huygens–Fresnel principle says that every point in the wavefront is also a source of spherical "wavelets", and these wavelets interfere with each other, and the sum of these spherical wavelets are the new wavefront (this also happens when the wave hits a interface, and mostly in these cases is usefull this principle). So, for example, when a plane wave hits a surface, in that moment the wave can "change" a spherical wave now (you can see this when a plane wave impacts a single slit). This also explains why a beam changes of direction when it hits a surface and is diffracted.

Answer 2

The Huygens-Fresnel principle states that every point on a wavefront is a source of wavelets. These wavelets spread out in the forward direction, at the same speed as the source wave. The new wavefront is a line tangent to all of the wavelets.