In the previous blog post I showed why a rainbow is semicircular using a GeoGebra simulation. But is it correct? To test the correctness I started to ask myself questions to interrogate the model. If you think about it, that's the way the scientific method works. A theory is developed to explain some experimental results and then tested against further experiments. It's also a way that we test our understanding of concepts: If the acceleration due to gravity is constant then a feather and a coin should fall at the same rate in a vacuum.

Here are my questions:

1. If a rainbow is supposed to be a semi circle how can you explain this?

At first I though that non semi circular rainbows must be fakes but heres a photo I took yesterday.

I know this isn't a fake. Maybe it's due to the angle of the sun. My model has horizontal rays so the sun is on the horizon, when I took that photo it wasn't. I will try to make a simulation to show this. In an aeroplane you can see circular rainbows.

Here is the simulation. he angles are kept the same but the angle of incident light is changed, see how the rainbow is only semicircular when the light is horizontal. You should be able to tell the time from the arc of a rainbow (EE idea).

2. According to the model the angle that the light makes to the eye is always the same for a given colour. This means that all rainbows should appear to be the same size. If different rainbows are photographed with the same camera they will all be the same size on the photograph. Are they?

Well they do seem to be I have never seen a tiny one or a huge one. The size on a photo depends on the lens and cropping but they do tend to always fill the scene.

This doesn't seem to be the case. I have made a simulation showing the rays for rainbows at different distances and the image in a single convex lens are not always the same size.

3. How can you get a double rainbow and is a triple rainbow possible?

According to the model there are 2 refractions maybe the second rainbow is due to the first refraction. This would only be possible with a high sun angle. So double rainbows should be quite flat.

This shows how a double rainbow is formed:

This one shows why the rainbow is red at the top and blue at the bottom:

and this one why the second rainbow has the colours reversed:

Note: The website Rainbow Symphony doesn't agree with me, I will have to investigate further.