What causes a rainbow?

Adam Bannister February 2, 2013 5

A highlight of a game drive can come in many shapes and forms. It could arise from watching a pride of lions hunting in the low light, a leopard perched high up in a tree, or a journey of giraffe sauntering through the long grass. It could be learning something useful about a tree or ticking a new bird on your life list. It could even be something as abstract as light and the way the light falls onto an object! Often, I find that my highlights appear in the lighting and ‘atmosphere’ of the drive. Nothing can beat the rays of golden light hitting the dead skeletons of the ancient Leadwood trees. Mysterious lighting, dark skies, energetic clouds and contrasting colours are all examples of how nature throws content onto the most beautiful of canvases.

What better way to punctuate a canvas then to throw a rainbow into the mix?

The Sand River is highlighted by the most gorgeous of rainbows. The perfect addition to a gin-and-tonic at Plaque Rock

But what exactly causes a rainbow?

Light is made up of a collection of many colors: red, orange, yellow, green, blue, indigo and violet. This is the reason that if you shine a white light through a prism you can produce a mini-rainbow on the other side. The reason for this is that the different colours of light are refracted, or bent, different amounts depending upon their wavelength (we perceive the different wavelengths as different colors). For example red is refracted the least, and violet the most. It is important to remember that light bends, or more accurately changes directions, when it travels from one medium to another. This happens because light travels at different speeds in different mediums.

Now in nature we just replace the prisms with raindrops (mist): they do the exact same thing. Light enters the raindrop, reflects off of the side of the drop and exits. In the process, it is broken into a spectrum of colours. You see red light come from a raindrop because that drop is at just the correct angle (about 42 degrees) between your eye and the sun so that the red light coming from the sun is refracted, reflected, and refracted again right into your eye. Blue light comes from another raindrop at a slightly different angle (as with all the other colours). All the raindrops that are at a certain angle between your eye the sun form a circle in the sky. That is why the rainbow is a circle.

$Image taken from Howstuffworks.com showing how the observers eye sees the light that is refracted and reflected back to your eye. The reason we see wide bands of colour is because we only see one color from each raindrop. You can see this in the above diagram.$

Image taken from Howstuffworks.com showing how the observers eye sees the light that is refracted and reflected back to your eye. The reason we see wide bands of colour is because we only see one color from each raindrop. You can see this in the above diagram.

The visible colour spectrum. For simplicity’s sake, we have only looked at red and violet, the colours of light on the ends of the visible light spectrum. The same applies to all the colours, it just changes depending on the different angles.

A rainbow is always directly opposite the sun from the observer’s perspective. This explains why rainbows are only seen when the sun is low in the sky, usually in the late afternoon or early morning.