What is a rainbow? It’s sunlight that has been bent, reflected and split into colours by countless drops of water, reaching your eyes at a precise angle. You don’t have to take that on trust, either, because you can make one yourself. On a sunny afternoon, stand with your back to the sun, set a garden hose to a fine mist and spray the water in front of you. Move around until the angle is right, and a small rainbow should appear in the spray.
That little rainbow demonstrates almost everything you need to know about how rainbows work. The sunlight has to come from behind you, the water droplets have to be in front of you, and the angle between them matters. It isn’t a trick of the eye. It’s optics happening in thousands of tiny drops of water.
What a rainbow actually is
A rainbow forms when sunlight passes through water droplets, and three things happen to the light inside each one. It’s refracted, or bent, as it enters the water. Some of it is reflected off the inside back surface of the droplet. And as it’s refracted again on the way out, it’s split into colours, because each wavelength bends by a slightly different amount, as the Met Office explains.
Sunlight may look white, but it contains a continuous range of colours. Violet light bends a little more than red, so the colours fan out and leave the droplet in slightly different directions. That’s why a primary rainbow always has red on the outside and violet on the inside.
The 42-degree rule
The single most important part of the explanation is the angle. Light that has made that refract, reflect, refract journey comes back out at roughly 42 degrees. So a primary rainbow appears about 42 degrees from the antisolar point, the spot in the sky directly opposite the sun. If you stand with your back to the sun, that point is roughly where the shadow of your head falls. Only droplets in the right part of the sky send their colours towards your eyes, and that’s the 42-degree rule behind the hose trick: sun behind, spray in front.
It also means a rainbow isn’t a fixed object. It’s made by particular droplets sending light to you. Someone standing a few metres away receives light from a different set of droplets and sees a slightly different rainbow, and if you move, your rainbow moves with you. That’s why you can never walk to the end of one: there’s no physical arc sitting anywhere for you to reach.
Why it’s curved, and why mornings and afternoons are best
The same angle explains the shape. All the droplets sitting 42 degrees from the antisolar point form a circle around it, so a rainbow is really a circle. From the ground, the horizon usually hides the lower part, leaving the familiar arch. From high up, in an aircraft or on a mountain, you can sometimes see much more of the circle. As the Met Office puts it, rainbows are actually full circles.
The height of the sun matters too. The centre of the rainbow’s circle sits directly opposite the sun, so as the sun rises, the antisolar point and the bow around it sink lower, as Physics World explains. Once the sun climbs more than about 42 degrees above the horizon, the whole rainbow drops below the horizon and you can’t see it from the ground.
That’s why rainbows are most common in the morning and late afternoon, and why you can stand in a midday shower in full sunshine and see nothing. There’s an Australian wrinkle here as well. In summer, the sun climbs especially high around the middle of the day, so the window for seeing a rainbow is even narrower, and the hose experiment works far better early or late.
Seven colours is a human idea
Most of us learned the rainbow as red, orange, yellow, green, blue, indigo and violet, but nature didn’t divide it into seven neat bands. Visible light is a continuous spectrum, with countless shades and no sharp line where one colour becomes the next. The Met Office notes that a rainbow contains far more hues than the human eye can distinguish.
The number seven came from Isaac Newton. According to Wikipedia’s article on rainbows, he first divided the spectrum into five main colours in 1672: red, yellow, green, blue and violet. He later added orange and indigo, making seven, to match the number of notes in a musical scale.
There’s nothing physically special about seven, which is also why many modern descriptions of the visible spectrum no longer treat indigo as a separate colour. The exact shades you see also depend on your eyes and the conditions, so it’s best to think of a rainbow as one continuous spread of colour that we’ve chopped into handy labels.
Double rainbows and the dark band between them
Sometimes a second, fainter rainbow appears outside the first. It forms when light reflects twice inside the droplets instead of once. That extra reflection sends the light out at a wider angle, about 51 degrees from the antisolar point, roughly 10 degrees outside the primary bow. It also reverses the colours, so the secondary bow has red on the inside and violet on the outside. The second bow is wider and fainter than the first.
If both bows are bright, you may notice that the sky between them looks darker than the rest. This is called Alexander’s band, after the Greek philosopher Alexander of Aphrodisias, who described it around AD 200. It isn’t a cloud or a shadow. As Wikipedia explains, the geometry of the two bows means light from drops in that part of the sky can’t reach your eyes, although it may help make a rainbow for someone standing somewhere else.
The “upside-down rainbow” is something else
You may have seen photos of a bright arc of colour high overhead, curving the wrong way like a smile. It’s often called an upside-down rainbow, but it’s usually a circumzenithal arc, which belongs to the family of halos rather than rainbows. It forms when sunlight passes through flat, six-sided ice crystals floating horizontally in high, thin cirrus cloud. It can only appear when the sun is lower than about 32 degrees, according to Wikipedia’s article on the arc.
So unlike a rainbow, it’s made by ice, not liquid water, and it appears near the top of the sky rather than opposite the sun. If you’re lucky enough to spot one, it really does look like a colourful smile overhead.
Rainbows have other relatives too. The Met Office describes pale fogbows, formed in the tiny droplets of fog, and faint moonbows, produced by moonlight rather than sunlight.
Make your own rainbow
Now the hose experiment makes complete sense, and it’s the easiest way to see the physics for yourself:
- Choose a sunny morning or late afternoon, when the sun is below about 42 degrees.
- Stand with your back to the sun.
- Set the nozzle to its finest mist and spray it in front of you, slightly upwards.
- Move the spray slowly until the colours appear.
Once you’ve found the bow, step from side to side and watch it move with you, because you’re changing the geometry between your eyes, the droplets and the sun. With plenty of fine mist and strong sunlight, you may even spot a faint second bow outside the first, with its colours reversed.
The hose itself isn’t the trick. It’s simply making millions of tiny droplets, each one acting like a miniature prism and mirror. The same thing happens wherever sunlight meets drops of water hanging in the air, from waterfalls, sprinklers and fountains to sea spray and passing showers, and the rule is always the same: sun behind you, water in front.
Sunlight, water and your own private bow
So a rainbow is sunlight bent, reflected and split into colours by countless water droplets, reaching your eyes from about 42 degrees around the point opposite the sun. It’s part of a circle the horizon usually hides, a continuous band of colour we’ve agreed to call seven, and a personal view made only for you. And if the next upside-down “rainbow” you see is high overhead under thin, wispy cloud, it’s probably an ice-crystal halo instead.
Sunlight puts on plenty of other shows in our sky, which I explored in why is the sky blue? and red sky at night, sailors’ delight?. You don’t need a laboratory to see any of it. Grab the hose, wait until the sun is low, put it behind you and make your own.
Sources
- Met Office: Rainbows, optical wonders
- Wikipedia: Rainbow
- Physics World: Rainbows, haloes and glories
- Wikipedia: Alexander’s band
- Wikipedia: Circumzenithal arc