![]() Let's start with a model of waves where particles don't have much kinetic energy. "Why are waves straight?" is the first question. After that, you get a complex pattern of isolated peaks. This frays the edges of the crest lines, progressively making them sorter, until they disappear entirely. But at both edges, you get the diffraction effects. In the middle, there's a well-defined crest line, and you still get the straight pattern. You can see how this makes sense when you look at broader slits. In thin slits, where there's no longer a crest line, this no longer makes sense, and that's why you get the diffraction. Wave crests travel orthogonally to the crest lines. Now if waves do not have memory, then how do they "know" how to travel in a straight line near the beach? Well, that's not really what happens. And that means it also doesn't remember in which direction it would need to travel. That means the wave inside the slit has no memory where it came from. Now we know that waves are really a local effect. Waves occur because the water movements aren't all in sync, nor could they be - how would all the water molecules know to reverse at the same time? So when you have water molecules traveling on opposite directions >^^<<<. ![]() But when you look at a fish in the water, it becomes clear that the water only sloshes back and forth. ![]() The first thing to realize is that waves only appear to travel. It doesn't really matter in what direction you consider: the waves will spread out into the 'harbour' because the water at the harbour mouth is moving.įrom this way of thinking, you begin to wonder why the waves out at sea are so straight! Ultimately it is because in that case you have oscillating water all along a long line, and so the water all along that long line is caused to move in synchrony.Īs I say, this is not a full mathematical answer, just an attempt to give you some intuition about the physics. Now the surface of the water nearby is going to bob up and down too, isn't it? And the ripples will spread out from there. ![]() So there is this water bobbing up and down in the small opening. the small opening in your diagram) the water there is caused to go up and down. As the waves reach the 'harbour mouth' (i.e. Suppose we are considering water waves, and imagine yourself sitting behind the barrier in the 'harbour' (at the lower part of your diagram), watching the waves approaching from 'out at sea' (i.e. For the full math, you can look up 'diffraction' and 'Huygens Principle' but here I will just post a quick observation that is enough to get a good physical intuition. ![]()
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