Rainbows

Rainbows are natural and beautiful phenomena that exemplify all of the above properties of light, namely refraction, dispersion and internal reflection. In order to see a rainbow it is necessary to look at a portion of the sky containing raindrops with the sun directly behind you. White light from the sun enters the raindrops, and gets refracted and dispersed inside the raindrop, as shown in the Figure belowl

  

Figure 10.8: Rainbow

When the dispersed light hits the back of the raindrop it gets internally reflected, and when it emerges it gets dispersed even more. Because it refracts more, the blue light always emerges from the raindrop above the red light. Consequently, only one colour reaches your eye from any given raindrop. What colour you see depends on the angle at which you look. In general you must look slightly higher up in the sky to see red light and lower to see blue light, so you what you see is a band of colour in the sky, with red on top and blue on the bottom, with all the colours of the rainbow in between. The reason rainbows appear as an arc in the sky is that the colours you see are determined by the angle that your line of sight makes relative to the position of the sun behind your head. As your look along the blue arc of a rainbow, for example, this angle remains constant.