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Scattering and diffraction

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这是个很好的问题，很多时候这两个概念是混淆的，这里只对电磁波方面谈一些个人浅显的认识。

散射和衍射是存在一些共同点的，至少描述他们的概念或者想象它们的物理图像时，都能想象出以上两个物理过程都涉及改变电磁波的原来传播方向。

它们之间有很多共同点，同时，也有很多区别。一般地，认为散射是非相干的过程，而衍射是一个相干的过程。这一点似乎很明确，我们谈及衍射总是绕不开干涉，但是谈及散射就很少谈及干涉。这从它们的数学表述上就可见一斑，衍射过程的描述都是从波动的角度出发，而散射的描述多是考虑粒子性或波动性。

除了以上从波动或者粒子的角度，或者从相干或非相干过程的角度之外，我之前看到过一句话，但是已经忘记出处了，大意是：“衍射是微观散射的宏观表象。”

我觉得这句话说得挺有道理的。散射一般认为电磁波在材料折射率非连续处发生传播方向的改变。一般根据不连续介质的大小分为一定不同种类的散射。总之，按照上一句话的意思，可以认为传播的电磁波遇到折射率非均匀的区域，例如遇到颗粒，导致了电磁波像四面八方传播，这就是散射的基本含义，而这些散射如果形成特定的干涉相长，例如电磁波遇到周期排布的颗粒，这些散射行为干涉相长，形成了特定方向的衍射，这样便更好地理解了“衍射是微观散射的宏观表象。”文献中的图解构了近远场的散射、衍射示意，和上述思想不谋而合。

感兴趣可以阅读文献，讨论电子/x-ray的散射与衍射（上图就是来自文献），可能对于题主说的本质上的理解会有一些帮助。

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