From sunlight to rainbow

White sunlight bends, splits into colors and reflects inside a raindrop. Follow in 14 scenes why that light bunches up at about 42°, why a rainbow is an arch, and where a double rainbow comes from.

Watch from the start in 3D

Chapter 1 · Light meets water

A rainbow needs two things: sunlight, and countless raindrops hanging in the sky.

1 The sky after rain
A rainbow needs sunlight and raindrops
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2 Sunlight is many colors
White sunlight holds many colors
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3 One raindrop
Zooming in on one drop among many
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Chapter 2 · Inside a raindrop

Light bends as it enters the drop, reflects off the back, and bends again on the way out.

4 Bending on the way in
Light slows down in water and changes direction
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5 Each color bends differently
Violet bends a little more than red
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6 At the back of the drop
Most light leaks out; only a little reflects
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7 Bending again on the way out
Each color leaves the drop in its own direction
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Chapter 3 · The secret of 42 degrees

Light leaving a raindrop bunches up at one angle. That angle decides where the rainbow appears.

8 Every entry height
A different entry height means a different exit angle
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9 Light bunches at 42°
Light piles up near the largest angle
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10 The shadow point
The sky shines 42° from your head’s shadow
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11 A different drop for each color
Red comes from higher drops, violet from lower ones
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Chapter 4 · Why is a rainbow round?

Join every direction at that angle and you get a cone. With no ground in the way, a rainbow is a full circle.

12 The cone and the horizon
The ground cuts the 42° cone into an arch
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13 Seen from the sky
With no ground in the way, a full circle
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14 Double rainbow
An outer rainbow made by light that reflects twice
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