To understand rainbows it is important to sort out what an observer is actually looking at.
- Rainbows only exist in the eyes of an observer.
- Every observer sees a different rainbow produces by a unique set of raindrops that happen to be in the right place at the right time.
- The individual raindrops that result in the appearance of a rainbow for one observer are always different from the raindrops that produce a rainbow for someone else.
- As an observer moves, their rainbow moves with them. Seen from a car window, the rainbow appears stationary whilst the landscape rushes past.
From an observer’s point of view
- Atmospheric rainbows appear to an observer as arcs of colour across the sky. From an aeroplane, a rainbow can appear as entire circles of colour.
- Even from the ground, it is easy to deduce that every rainbow has a centre point around which the arcs of a rainbow are arranged.
- The exact position in the sky where an atmospheric rainbow will appear can be anticipated by working out where its centre will be.
- The centre of a rainbow is always on an imaginary straight line that starts at the centre of the Sun behind you, passes through the back of your head, out through your eyes and extends in a straight line into the distance.
- The eyes of an observer are always aligned with the rainbow axis.
- To an observer, the rainbow axis appears as a point, not a line, and that imaginary point marks the centre of where every rainbow will appear.
- The idea that a rainbow has a centre corresponds with what an observer sees in real-life.
- The idea of a rainbow axis or anti-solar point corresponds with a diagrammatic view showing the scene in side elevation.
Looking for rainbows
- To work out where a rainbow might appear:
- Turn your back on the Sun.
- If you can see your shadow, look at the head. The axis of the rainbow runs from the Sun behind you, through your eyes and through the head of your shadow. Imagine where your eyes might be in your shadow. If a rainbow appears that point will be its centre.
- If you can’t see your shadow, just try and imagine the line from the Sun, passing through your head and then extend it away from you till it reaches the landscape. At whatever point it touches, that will be the centre.
- Unless you are in a plane, the centre point is always below the horizon so on the ground or in the landscape in front of you.
- Now, with the Sun behind you spread out your arms to either side or up and down at 450 from the centre point.
- Swing them round like the blades of a windmill. That is where your primary rainbow will appear.
- Every observer has a rainbow axis and a centre-point on that axis that moves with them as they change position. It means that their rainbow moves too.
- The centre of a secondary rainbow is always on the same axis as the primary bow and shares the same anti-solar point.
- To see a secondary rainbow look for the primary bow first – it has red on the outside. The secondary bow will be a bit larger with violet on the outside and red on the inside.
Rainbows as discs of colour
- Close consideration of why rainbows appear as arcs or circles can be explained by the idea that an observer is looking at superimposed, concentric discs of colour.
- Think in terms of each observed band of colour within a rainbow forming on the edge of a separate coloured disc.
- The area close to the circumference of each disc produces the most intense and brilliant colour.
- The intensity of each colour drops sharply away from the circumference of its disc and towards the centre.
- The observed colour of each disc corresponds with the band of wavelengths that produces it.
- The fact that we see distinct bands of colour in a rainbow is often described as an artefact of human vision.
- Each disc contributes small amounts of its own colour to the area towards the shared centre of the six concentric discs making the sky appear lighter.