How Rainbows Form

Overview

Steps Involved

Overview

Figure 1:
Basic diagram showing formation of rainbow.

Note:
Angles not to scale.

The formation of a rainbow involves a series of physical phenomena - reflection, refraction, dispersion and total internal reflection.
The occurence of each of these is due to the interaction of light with air and water and the boundaries between them.

Enlarged view of diagram.

The Steps Involed in the Process

Figure 2:

Ray digram - light through raindrop

1. Light from Sun strikes raindrop
2. Some of the light is reflected
3. The rest of the light is refracted
4. Light splits into component colours
5. Reflected at rear of raindrop (TIR)
6. Refracted again as it leaves raindrop
7. Colours are further dispersed

Click on the links above for more detail on ech step.

1. Light from sun strikes raindrop.

White light from the

2. Some of the light is reflected.

It is possible to see through a glass window but, at the same time, see your own reflection. This is because the window both transmits and reflects light. Water can do this too - that is why you can see a reflection in a pool of clean water and also see the bottom.

When light from the sun hits a water droplet, some of the light is reflected. This light will obey the Law of Reflection.

3. The rest of the light is refracted.

The light that is not refracted crosses the air-water interface (boundary layer). When this happens it slows down because the water is more dense than the air. The reduction of speed cause the path of the light to bend - this is called refraction.

In this case the path of the light rays bends toward the normal line.

4. White light splits into component colours.

White light is made up of a spectrum of colours, each with its own wavelength. Different wavelengths travel at different speeds and when they encounter a change to medium that is more dense or less dense, the speeds are efected by different amounts. Hence, the colours separate. This phenomenon is know as Dispersion.

5. Light is reflected at rear of raindrop (TIR).

At the rear of the raindrop, the light hits the water-to-air interface. If the angle of incidence is greater than the critical angle, Total Internal Reflection will occur. A rainbow will only be seen if this happens, otherwise the light will continue out the other side of the raindrop and continue to move away from the would-be viewer.

6. Light is refracted again as it leaves raindrop.

Just as the light changed speed as it entered the raindrop, its speed changes again as it leaves. Here, the light is moving from a more dense medium (water) to a less dense medium (air). As it does so, it speeds up and its path bends. In this case the path of the light rays bends away from the normal line. This is another example of refraction.

7. Colours are further dispersed.

As the rays are refracted once again, the various wavelengths are effected to different extents. The overall result of this is increased separation of the component colours of white light. This is Dispersion.