Did you ever notice that you usually see the rainbow when the sun comes out after a shower?  Well, that is because light and water are the two main ingredients you need to make a rainbow.  What happens is this: sunlight travels from the air and hits the water droplets left over by the rain in the atmostphere.  Since sunlight is made up of light of different frequencies, the different frequencies are refracted at different angles into the raindrop.  So white sunlight is scattered out into the familiar 7 colours.  When the different frequencies hits the inside surface of the back of the raindrop, they are reflected.  Then they emerge from the raindrop into the air, and they are refracted once again.  The path of the light rays is illustrated in red in Figure 1.

Home Page

The Physics of Rainbows


More About Us

Figure 1:  Ray of light entering and exiting a rain drop

Related Sites

Figure 2:  Angle of Minimum Deviation

Aside from requiring the presence of the sun and water droplets in the atmosphere, in order to see a rainbow, the sun must also be directly behind the person who is viewing it.  Moreover, the person's line of sight must make an angle of approximately 42 degrees with the incident sunlight.  Otherwise the rainbow would not be seen (see Figure 2).