A shower with a colourful bonus
A rainbow forms when sunlight bends as it enters water droplets, reflects inside them and bends again as it leaves. Different colours bend by different amounts, so they spread out. NOAA follows that journey through a droplet.
The rain has interrupted your walk. Then the Sun appears and the sky seems to apologise with a rainbow. The ingredients are ordinary; their arrangement does the clever part.
One droplet, three changes of direction
Light travels more slowly in water than in air. When it enters a droplet at an angle, its path bends. That bending is called refraction.
Some light then reflects off the inside of the droplet, like a small bounce, before bending again on its way out. The National Weather Service explains this route. It is not simply sunshine reflecting off the outside of rain.
The colours were there all along
As the first lesson showed, sunlight contains a range of colours. A droplet separates them rather than making new ones. This separation is called dispersion.
In a primary rainbow, red sits on the outside of the arc and violet on the inside. We often name seven colours, but the spectrum blends continuously. The Met Office explains why seven is a useful description, rather than seven separate stripes.
Why the rainbow will not wait for you
For an ordinary daytime rainbow, look towards rain with the Sun behind you. Your viewpoint matters: the rainbow is an optical effect, not a painted arch fixed to a field.
NOAA explains that the full shape is a circle; from the ground, we usually see only part. In the right conditions, a view from above can reveal more.
Move and the view changes with you. That elusive end is not playing a particularly mean game of hide-and-seek.