< b >短答:< / b >彩虹的形成是当光进入一滴水或冰晶体折射和反射回一个观察者的眼睛。< b >长答:< / b >光速在不同速度取决于所涉及的媒体;它通过水比通过空气传播慢,例如。当光进入雨滴或冰晶体,它首先被折射。然而,并不是所有频率的光折射的角度。更短的波长的颜色(如蓝、靛、紫)得到折射不到时间越长波长(红色)。这个利差的颜色就像穿过棱镜。然后光被反射回来的雨滴,再次折射通过雨滴,飞行到观察者的眼睛。原因是圆形的光折射和反射回了观察者,这样做在一个特定范围的角度,40到42度。为了看到彩虹,太阳(或其他光源)必须直接背后的观察者。 A straight line is formed from the light source, the observers eyes, and the center of the rainbow. All rainbows are actually circles. 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