Rainbows

 

We all know that to see a good rainbow we need rain in front of us and sunshine in the back. Then this beautiful arch manifests itself, violet on the inside to red on the outside.

Is it really a circle? No, it is a 3 dimension object, a cone in fact. It all happens where the rain is. Millions of water drops in the air, each one acting as a tiny spectroscope: the white sunlight beam enters the drop on one side and gets refracted. What does that mean? It gets deflected by an angle that depends on the colour: violet is the most deflected, green is average and red is the least deflected. Then the light gets reflected on the back of the drop and comes back to the front where it gets refracted again. In total, the outgoing bean forms an average angle of 42 degrees with the incoming beam. 

 So you see the rainbow as a circular arc, but in fact it is a cone. The axis of the cone is the straight line defined by the Sun and your eye, which is where the cone apex is. And the half-angle of the cone is… the magic number, 42 degrees!

Next time you see a rainbow, see if you can locate the shadow of your head on the ground in front of you. You will find that it is bang smack at the centre of the rainbow. It shows you exactly where the axis of the cone is.

References:

ABC:         https://education.abc.net.au/newsandarticles/blog/-/b/2626014/curious-kids-why-are-rainbows-round-

Wikipedia: https://en.wikipedia.org/wiki/ROYGBIV