Raindrops as prisms
How Does a Rainbow Work?
Every raindrop bends sunlight into its colours and sends them back towards you.
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Step 1 of 5
You need sunshine and rain at the same time.
The Sun must be behind you and the rain in front. That is why rainbows appear as a shower is ending and the sky is clearing.
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Step 2 of 5
Light bends when it enters water.
Light slows down going into a droplet and changes direction. Each colour bends by a slightly different amount, so they fan apart.
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Step 3 of 5
It bounces off the back of the drop.
The fanned-out light reflects off the droplet's far side and comes back out towards you — which is why the Sun must be behind you.
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Step 4 of 5
The angle is always 42 degrees.
Light leaves the drop at a fixed angle, so a rainbow always appears as a circle 42 degrees from the point opposite the Sun.
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Step 5 of 5
Each drop sends you only one colour.
The drops sending you red are higher in the sky than the ones sending you violet. The bow is a map of which drop is where.
A rainbow is really a full circle. From a plane you can sometimes see the whole ring.
The short version
Each raindrop bends, splits and reflects sunlight back at a fixed 42-degree angle, building a circle of colour.
Try it yourself
Stand with your back to low sun and spray a hose as fine mist. Your own rainbow appears, and moves when you move.
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