来自《耶鲁大学开放课程：新生有机化学 第5集 第5集》

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  what directions will you get reinforcement in?
  在哪个角度可以观察到增强?
• 
  
  Well, there are two directions that you're guaranteed,
  有两个角度 是一定可以观察到的
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  no matter what the spacing is
  无论电子间距大小
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  and that's this.
  如图所示
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  Here the light comes in and goes out.
  光波入射进来 再出射
• 
  
  So the direct beam, you have the same path,
  在光波原始方向上 光程是相同的
• 
  
  because one of them gets hit earlier,
  因为 尽管一束光被碰撞得早一些
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  but has a longer path coming out,
  相应的出射光程却长
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  and the other one gets hit later but has a shorter path coming out.
  另一束光碰撞较晚 但出射光程较短
• 
  
  So they're guaranteed to be exactly in-phase
  所以在沿入射光传播的方向上
• 
  
  if they're scattering straight ahead.
  两者必然是同相位的
• 
  
  But only a little of the light is getting scattered.
  但是 入射光只有一小部分被散射