White Light Is a Mix of Colours (How Rainbows Form)
Follow a beam of sunlight through a prism and a raindrop to discover that white light hides every colour of the rainbow.
Rainbows and a puzzle
Think of the last rainbow you saw. The rain had just stopped, the Sun came out, and a glowing arc of colours appeared in the sky. But where did those colours come from? A moment before, the sunlight looked plain white, and nobody painted the rain. In this chapter we will follow the light step by step, and ask but why at every turn, until the rainbow makes complete sense. Our big discovery will surprise you. White light is not one colour at all.
Light bends in water
In the last chapter we saw that light travels in straight lines, but bends when it passes from air into water or glass. Put a pencil in a glass of water and it looks broken at the surface. That bending is called refraction. It happens because light slows down in water and glass. Imagine a toy car rolling at an angle from a smooth floor onto a carpet. One wheel reaches the carpet first and slows, so the car turns. Light turns in a similar way.
Newton's prism
Over three hundred and fifty years ago, Isaac Newton let a thin beam of sunlight into a dark room through a small hole, and placed a prism in it. A prism is a triangle shaped block of clear glass. On the far wall, the white beam spread into a band of colours, called a spectrum. Newton named seven colours, including indigo, partly to match the seven notes of a musical scale. Really, the colours blend smoothly into each other, with no sharp edges between them.
Each colour bends differently
Why does the glass spread the colours apart? Because each colour bends by a different amount. Red light bends the least. Violet light bends the most. The other colours fall in between, in order. So when white light goes in, the colours come out travelling in slightly different directions, and they fan out. Splitting light into its colours like this is called dispersion. The deeper reason is that inside glass, violet light is slowed down a little more than red light, so it turns more.
Proof the colours were there
But wait. Maybe the prism adds the colours, like dye added to water? Newton tested this idea. He let just one colour, say green, pass through a narrow slit into a second prism. The green light bent, but it stayed green. No new colours appeared. Then he used a second prism turned the other way to bring all the colours back together, and out came white light again. So a prism does not make colours. It only sorts out colours that were mixed in the white light all along.
What makes colours different
So how do red and violet light differ? Scientists describe light as a wave, like ripples on a pond. The distance from one ripple top to the next is called the wavelength. Red has the longest wavelength we can see, about seven hundred nanometres, and violet the shortest, about four hundred. A nanometre is one billionth of a metre. Past each end lies light we cannot see, called infrared and ultraviolet. The wave picture is not the whole truth, because light also comes in tiny packets called photons.
A raindrop is a prism
Now back to the sky. Each raindrop is a tiny ball of water, and water bends light just as glass does. Follow one ray of sunlight. First, it bends as it enters the drop, and the colours start to separate. Second, it bounces off the inside of the back of the drop. Bouncing back like this is called reflection. Third, it bends again as it leaves, spreading the colours even further apart. The light comes back out towards the side where the Sun is.
Why it is a bow
Light leaving the drops comes out most strongly at a special angle, about forty two degrees for red and about forty degrees for violet, measured from the shadow of your own head. So every drop that sits forty two degrees from that point sends red light into your eye. All those drops together make a circle around your shadow. Drops slightly closer in send violet. That is why red is on the outside and violet on the inside. The ground usually hides the lower part of the circle.
How to see one
Now you can predict rainbows. You need three things: the Sun behind you, raindrops in front of you, and a Sun that is low in the sky. If the Sun is higher than about forty two degrees, the whole bow sits below the horizon. That is why rainbows are most common in the morning and late afternoon. From an aeroplane or a mountain you can sometimes see a full circle. You can even make your own with a garden hose spraying fine mist, with your back to the Sun.
Double rainbows
Sometimes a second, fainter rainbow appears outside the first. Here, light bounces twice inside each drop before leaving. Every bounce loses some light, so this bow is dimmer. The extra bounce also flips the order, so the second rainbow has red on the inside and violet on the outside. It sits at about fifty one degrees. Between the two bows the sky looks darker, because neither bow sends light to you from there. This darker strip is called Alexander's dark band.
Common mistake
A very common mistake is thinking a rainbow is an object standing in one place, like a colourful bridge with ends you could walk to. It is not. A rainbow is light reaching your eye from many drops that sit at just the right angle. If you walk towards it, different drops do the job, and the rainbow moves along with you. So nobody can ever reach its end. Even two friends standing side by side get light from different drops, so each sees their own rainbow.
Why a leaf looks green
This also explains the colours of everyday things. A green leaf is lit by white sunlight that contains every colour. The leaf soaks up, or absorbs, most of the red and blue light, and uses it to make food. It bounces back, or reflects, mostly green, so green reaches your eye. A white shirt reflects all colours. A black shirt absorbs most of them, which is why it warms up in the Sun. Lit only by red light, a green leaf would look dark.
Mixing light back to white
If white light is a mix, can we build white ourselves? Look at a screen through a magnifying glass and you will see tiny dots of only red, green and blue. Our eyes have three kinds of colour sensing cells, called cone cells, so these three lights are enough to fool them, and together they look white. Mixing paints is different: paint absorbs light, so many paints mixed give mud, not white. Exactly how the brain turns signals into the feeling of colour is still being studied.
Explain it back
Let us put it all together. White light is a mix of all the colours. Glass and water bend each colour by a different amount, which spreads them apart. In a raindrop, light bends in, reflects off the back, and bends out again, and each colour leaves at its own angle, making a circle of colour around your shadow. Now try teaching it to a friend. Can you explain why red is on the outside, and why nobody can ever reach the end of a rainbow?