A secondary rainbow appears when sunlight is refracted as it enters raindrops, reflects twice off the inside surface, is refracted again as it escapes back into the air, and then travels towards an observer.
- A secondary rainbow always appears alongside a primary rainbow and forms a larger arc with the colours reversed.
- A secondary rainbow has violet on the outside and red on the inside of the bow.
- When both primary and secondary bows are visible they are often referred to as a double rainbow.
- A secondary rainbow forms at an angle of between approx. 50.40 to 53.40 to its centre as seen from the point of view of the observer.
- A secondary bow is never as bright as a primary bow because:
- Light is lost during the second reflection as a proportion escapes through the surface back into the air.
- A secondary bow is broader than a primary bow because the second reflection allows dispersing wavelengths to spread more widely.
- The axis of a rainbow is an imaginary line passing through the light source, the eyes of an observer and the centre-point of the bow.
- The space between a primary and secondary rainbow is called Alexander’s band.