How do rainbows form?
04 March 2016
Rainbows have appeared in art and legend throughout history and continue to amaze when they appear in splendid arcs across the sky. But what’s the science behind these dazzling displays? And is it possible to find the end of the rainbow?
Why do rainbows occur?
Rainbows are optical phenomena that occur when sunlight and rain combine in a specific way.
When white sunlight enters a raindrop, it changes direction (refracts) at the boundary of the raindrop. As different wavelengths within the sunlight refract at slightly different angles, the colours spread out and separate. Some of the light bounces off the back of the raindrop (reflection) and as this light comes back out of the raindrop it refracts again, spreading the colours out further.
Sunlight refracting and reflecting through a raindrop
When an observer looks away from the sun and towards rain, there is an arc of the sky in which the angle between the sun, raindrops and the observer is 42°. The observer will see just the red light reflected from raindrops in that arc. From raindrops a little lower in the sky, where the angle is 41°, you see just green light reflected; and from drops lower again (40°) you see violet light reflected.
How big is a rainbow?
Depending on how high the sun is, you might see a shallow arc or closer to a full semicircle; but as the angle between the sun, the raindrops and the observer is always the same (42°) the diameter/curvature of the arc will always be the same—however near or far the raindrops are to the observer. The reason some rainbows seem wider than others is that your brain compares the arc to other objects in view and makes an assumption about how far away the rainbow is. For example, if you see a rainbow over mountains you might get the impression it's a kilometer or more across, but if you see a rainbow in a sprinkler spray it would only seem a meter across. If you saw both at once, though, you'd see they have the same diameter.
What are the best conditions to see a rainbow?
Bright sunlight, suspended droplets of water (rain, spray or fog), and the right angle of the sun are essential conditions to see a rainbow. For rainbows to be visible in the sky, the sun must be behind the observer, at 42° elevation or lower. As such, rainbows are more likely to be seen towards the end of the day when the sun is lower in the sky.
As well as traditional sightings in the sky, rainbows may also be seen in the mist of water around a waterfall, backyard water sprinkler, or hose.
Can you get multiple rainbows?
Multiple rainbows are possible but they’re less common. Double, triple, or even quadruple rainbows can occur when sunlight is reflected more than once inside a water droplet—exiting the drops at steeper angles, so visible higher in the sky. However, the non-primary rainbow(s) will be dimmer, as energy is lost as light bounces around inside water droplets.
These stunning displays of colour are widely shared in the news and on social media.
Double rainbow, Sydney, 17 June 2015. Photos courtesy of Bureau of Meteorology staff.
So where’s the end of the rainbow?
Rainbows are optical phenomena—there’s nothing to find or catch at the end. As you move, your view of the sky changes relative to the sun. A rainbow will either appear to move with you, as you start to see colours reflected from rain in another part of the sky, or disappear if there’s no rain there or not enough sunlight reaching it.