Rainbows are caused by both the reflection and refraction of light. As rain falls, some of the sunlight shining on raindrops gets reflected, as each raindrop acts like a tiny mirror. Each raindrop also acts as a tiny prism, so the reflected light is also refracted. The amount of refraction depends on the wavelength of the light. Each colour in the visible light spectrum has a slightly different wavelength, which causes each colour to be refracted differently in a raindrop. That is why we can see the different colours in a rainbow. The picture above was taken early yesterday morning in Salthill – you can see the Burren hills in the background. It shows a few different phenomenon relating to rainbows. The first is obviously the rainbow itself – the really bright one.  Secondly, a second fainter rainbow can be seen further out (with the colour pattern reversed) – that is a secondary rainbow caused by secondary refraction within the raindrops. It is not an unusual occurrence – I’ve seen double rainbows lots of times. What is unusual is the third phenomenon visible in the picture – what looks like a middle rainbow at a different angle to the other two. This is a reflection rainbow, caused by sunlight reflecting off a large body of still water – in this case, a very calm Galway Bay.  The sunlight  reflects first off the sea and then hits the falling raindrops from below, causing a different refraction [and therefore different shaped rainbow] to occur. Reflection rainbows always intersect with ‘ordinary’ rainbows at the horizon. In this picture, there is actually a secondary reflection rainbow, though it is very faint [it also has the colour band in reverse order].

The beauty of a rainbow is literally in the eye of the beholder – the location of a rainbow is dependent on the observer. A rainbow always appears directly opposite the sun, so an observer always has his/her back to the sun when looking at a rainbow. If you were to draw a triangle between yourself, the top of your shadow and a point on the rainbow, the angle facing you would always be between 40 and 42 degrees, for the primary rainbow and between 50 and 53 degrees for the secondary rainbow..  For reflection rainbows, the angles are the same [for primary and secondary rainbows] – the shape is different because the light is coming from the ground [i.e. a still body of water] rather than directly from the sun in the sky.

You can see the full rainbow in this picture [click on link] which is a panorama of 4 images merged together in Photoshop. A very good explanation of rainbows is available on Youtube click for Part 1 or Part 2.