简单的回答:当光进入一滴水或冰晶，经过折射并反射回观察者的眼睛时，彩虹就形成了。

长回答:光以不同的速度传播，这取决于所涉及的介质;例如，它在水中比在空气中传播得慢。当光线进入雨滴或冰晶时，首先会发生折射。然而，并不是所有频率的光都以相同的角度折射。波长较短的颜色(如蓝色、靛蓝色和紫色)比波长较长的颜色(红色)折射得更多。这就像穿过棱镜一样分散了颜色。然后，光线从雨滴的背面被反射，在穿过雨滴时再次发生折射，并到达观察者的眼睛。它是圆形的原因是因为光被折射并反射回观察者，在一个特定的角度范围内，40到42度。为了看到彩虹，太阳(或其他光源)必须在观测者的正后方。从光源、观察者的眼睛和彩虹的中心形成一条直线。所有的彩虹实际上都是圆形的。 We usually see it as an arc because the earth intercepts the circle. (BTW, because of this, no two people, even standing shoulder to shoulder, can see the same exact rainbow. Each rainbow is being created by different raindrops.) If you're ever in an airplane passing over a cloud with the sun above you (as opposed to near the horizon), you may see a complete rainbow. It's pretty awesome. 

 Here's an illustration:
[![from: https://strongphysics.wikispaces.com/ch29_egtk][1]][1] [1]: http://i.stack.imgur.com/0Ooxh.jpg