Why do waves diffract, really?

[yeet]

So this isn't really homework help at all, I'm currently doing a biology undergrad, this is something I saw in high school, but I never really understood.

Basically, I've been given the basic explanation of the Huygens principle, but it doesn't really make sense to me. It seems to me like this principle is saying that, for some reason, the distance at which wave front 1 is situated from wave front 2 has some sort of magical influence on wave front 2 and the way it interacts with some obstacle. Obviously, that's not actually the case, and what must happen is that the two wave fronts interact with each other in some way. However, if that's the case, shouldn't amplitude matter, too?

If someone has a clear, but more detailed explanation, I'll take it.

 

[Drakkith]

I think the key idea of diffraction is that the wavefront can't be discontinuous. That is, it can't contain gaps or instantaneous jumps in its amplitude between two adjacent points. Imagine a small column of water with completely straight, vertical sides, that is traveling through the water. These perfectly straight sides represent discontinuities in the wavefront since the amplitude of the wave goes from maximum to zero instantly. This can't physically happen, as gravity would pull down on the column and the discontinuity would be destroyed as the column turns into a proper wave. This, of course, is exactly what happens in diffraction, except that that such a discontinuity never existed in the first place. Instead, large, quick changes in the amplitude of the wave end up being smoothed out as the wave travels. So when a wave passes by a barrier or through an aperture, the wavefront must diffract in order to avoid discontinuities.

An analogous effect occurs in other types of waves. EM waves cannot have instantaneous jumps in their field vectors (the things that represent the strength and direction of the force of the EM field) and so also undergo diffraction. Sound waves cannot experience discontinuities in their pressure. The pressure from the air in one part of the wavefront pushes on adjacent areas, preventing any discontinuity and causing diffraction as the wave travels.

It seems to me like this principle is saying that, for some reason, the distance at which wave front 1 is situated from wave front 2 has some sort of magical influence on wave front 2 and the way it interacts with some obstacle.

Nothing magical about it. Look at a sound wave. The pressure from the air in one part of the wavefront pushes on the surrounding areas, and in turn they are pushing on adjacent areas too. We can model this as the wavefront being made up of an infinite amount of points, with each point generating its own spherical wave that all interact to form the single, large wavefront that is observed.

 

[sophiecentaur]

There is nothing magical about diffraction. If you acknowledge that a wave from anything but a (unachievable) point source will produce a non uniform pattern of energy density at some destination due to all parts of the wave front. That pattern can be described (predicted) by using diffraction theory.
Nothing is “really” anything. We just call the effect Diffraction.