A rainbow is an optical effect produced by illuminated droplets of water. Rainbows are caused by reflection, refraction and dispersion of light in individual droplets and results in the appearance of an arc of spectral colours.
- Atmospheric rainbows only appear when weather conditions are ideal and an observer is in the right place at the right time.
- Waterfalls, lawn sprinklers and other things that produce water droplets can produce a rainbow.
- A rainbow is formed from millions of individual droplets each of which reflects and refracts a tiny coloured image of the Sun towards the observer.
- It is the dispersion of light as refraction takes place that produces the band of rainbow colours seen by an observer.
- If the sun is behind an observer then the rainbow will appear in front of them.
- When a rainbow is produced by sunlight, the angles between the sun, each droplet and the observer determine which ones will form part of the rainbow they see and which colour each droplet will produce.
- Rainbows always form arcs around a single centre point (anti-solar point) with each colour at a slightly different angle to an observer.
- The axis of a rainbow is an imaginary line drawn between the light source and the anti-solar point of a rainbow with the observer in between.
- If you can see your own shadow and a rainbow at the same time then the rainbow always has the shadow of your head as its centre.
- Seen from the air a rainbow can appear as a complete circle. It is only because the ground around the observer gets in the way that a rainbow produced by sunlight is reduced from a circle to a semi-circle or an arc.
- The sky inside a rainbow is brighter than on the outside because raindrops scatter diffuse light of every wavelength inwards towards the centre but none is directed outwards.
- When an observer sees a single rainbow, red appears on the outside, followed by orange, yellow, green, and blue, with violet on the inside.
- When an observer sees a double rainbow, the secondary rainbow is outside the first and forms a wider, paler band of colours with violet on the inside.