Jenny Harris, of Balclutha Primary School asks :-
I photographed a triple rainbow. Is it rare or just unusual for this to happen and what causes this?
John Campbell, a physicist at the University of Canterbury who has an interest in light and colour in the atmosphere, responded.
What a superb photo. The normal rainbow is when light goes into a raindrop, gets reflected off the back surface and comes out again, back towards the Sun. We see it by looking away from the Sun. This rainbow is basically because the angle between the the line from the Sun through our eyeball and the back scattered bow has a maximum value. ie inside the bow is lighter than outside the bow.
The colour of the rainbow is because this angle is slightly different for different colours.
There is a second bow where the light has reflected off the back side of the raindrop twice, rather than once. The second bow is also seen when looking away from the Sun. It is much weaker and seldom seen because on reflection some of the light exits the raindrop. It is for the same reason that the first bow is much weaker than the Sun.
For the second reflection, there is a minimum angle possible, hence the area between the two bows is dark compared to inside the first bow and outside the 2nd bow. You can see that in your photo of the two bows.
There are 3rd and 4th bows (3 and 4 reflections inside the raindrop) but they appear to come from the direction towards the Sun so are never seen. Each reflection is a much weaker bow than the one before.
The 5th bow (5 reflections inside the rain drop) is also seen looking away from the Sun but is so weak it is never seen with the naked eye.
The striations inside the main bow are due to interference of light in small raindrops, not a rainbow as such. Their appearance and colours allow the drop size to be measured.