Rainbow | lightcolourvision.org

A rainbow is an optical effect produced by illuminated droplets of water. Rainbows are caused by reflection, refraction (bending) and dispersion (spreading out) 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 air-borne water droplets can produce a rainbow.
An atmospheric rainbow is formed from countless individual droplets each of which reflects and refracts a tiny coloured image of the Sun towards the observer.
As white light passes through water droplets, refraction causes the light to disperse and separate into the different 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, the colour each droplet will produce and the sequence in which they appear.
Rainbows always form arcs around a single centre point (anti-solar point: the point in the sky directly opposite the sun) with each colour merging into the next one at a slightly different angle as seen from the point of view of 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 shadow of your head is always at the centre of the circle or arc of the rainbow.
Seen from the air a rainbow can appear as a complete circle, but from the ground, it always appears as a semicircle or arc because the ground around the observer gets in the way.
The sky inside a rainbow is brighter than on the outside because raindrops scatter diffuse light of every wavelength towards its centre whilst almost 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.