How Our Eyes See Colour (How Rainbows Form)
Follow sunlight through raindrops and into your eyes to discover where rainbows and colours really come from.
A Puzzle in the Sky
Imagine a sunny afternoon just after a rain shower. You turn your back to the Sun, and there it is: a glowing arch of colour. You can even make a small rainbow by spraying water from a hose on a sunny day. But where do those colours come from? Water is clear, and sunlight looks white. And why does a banana look yellow while a leaf looks green? In this chapter we will follow light step by step, from the Sun, through raindrops, and into your eyes and brain.
Light Travels as Waves
Scientists have found that light travels like a wave, a bit like ripples spreading across a pond. The distance from the top of one ripple to the top of the next is called the wavelength. Light waves are astonishingly tiny. Red light has a wavelength of about seven hundred nanometres, and a nanometre is one billionth of a metre. Violet light is shorter, about four hundred nanometres. Light also behaves like tiny packets of energy called photons, but the wave picture is all we need today.
White Light Is a Mixture
About three hundred and fifty years ago, Isaac Newton let a thin beam of sunlight into a dark room and passed it through a triangle of glass called a prism. Out came a band of colours, called a spectrum. Some people thought the glass was adding colour. So Newton sent one single colour through a second prism, and it stayed the same colour. Then he combined all the colours and got white again. White light is a mixture. Newton named seven colours, but really they blend smoothly into each other.
Why Light Bends
When light passes from air into glass or water, it slows down. In water it travels at about three quarters of its speed in air. If it hits the surface at a slant, it changes direction. Picture a toy car rolling at an angle from a smooth floor onto carpet: one front wheel slows first, and the car swings round. That is only a simplified picture, but this bending is called refraction. Violet light slows a little more than red, so it bends more. This spreading of colours is called dispersion.
Inside a Raindrop
Now we can solve the rainbow. Small raindrops are nearly perfect balls of water, not tear shaped, and each one acts like a tiny prism. Follow one ray of sunlight. First it enters the drop and bends, so the colours begin to spread apart. Next it hits the back of the drop and some of it bounces back, which is called reflection. Then it leaves the front and bends again, spreading the colours further. The light comes out on the same side it went in, angled downwards towards you.
The Shape of a Rainbow
Each drop sends red light out at about forty two degrees from the direction the sunlight travels, and violet at about forty degrees. So red reaches your eye only from drops at that angle around the point directly opposite the Sun, which is where the shadow of your head falls. Those drops form a circle, so a rainbow is a curved arc, red outside and violet inside. Sometimes light reflects twice inside the drops, making a fainter second rainbow above, with its colours in reverse order.
Common mistake
Many people think a rainbow is a real object standing in one place, like a bridge with an end you could walk to. It is not. A rainbow is light from millions of drops arriving at your eyes at just the right angle. If you walk towards it, different drops take over, and the rainbow moves with you. A friend standing beside you sees light from slightly different drops, so you each see your own rainbow. That is why nobody can ever reach the end of a rainbow.
Why Things Have Colour
So why is a leaf green? Sunlight landing on a leaf contains every colour. The leaf contains a substance called chlorophyll, which soaks up, or absorbs, much of the red and blue light to make food for the plant. Much of the green light bounces off, which we call reflection, and that green light travels into your eyes. A ripe strawberry reflects mostly red. White paper reflects nearly all colours, and black cloth absorbs nearly all of them, which is why black clothes warm up in sunshine.
Journey Into the Eye
Now let us follow light into your eye. First it passes through the cornea, a clear dome at the front that does most of the bending. Then it goes through the pupil, the dark opening in the middle, which widens in dim light and shrinks in bright light. The coloured ring around it is the iris. The lens then fine tunes the focus. Finally the light lands on the retina, a thin layer at the back packed with light detecting cells. The image there is upside down, and your brain sorts that out.
Rods and Cones
The retina has two main kinds of light detecting cells, named after their shapes. Rods are long and thin. There are about one hundred and twenty million of them, and they are very sensitive, so they let you see in dim light, but they cannot tell colours apart. That is why a garden at night looks grey. Cones are shorter, and there are about six million of them, crowded into a small spot at the centre of the retina. Cones need brighter light, and they give us colour vision.
Three Kinds of Cones
Most people have three kinds of cones. One kind responds most to short wavelengths, which we see as blue. One responds most to medium wavelengths, around green. One responds most to longer wavelengths, towards yellow and red. People call them blue, green and red cones, but that is a nickname, because their ranges overlap a lot. When light arrives, your brain compares how strongly each kind is signalling. Yellow light makes the red and green cones both signal strongly, and your brain reads that pattern as yellow.
Colour Is Made in the Brain
Here is a surprising idea. Light itself is not coloured; it just has different wavelengths. Colour is something your brain builds from signals sent along the optic nerve. That is how a phone screen fools you. It has no yellow dots, only tiny red, green and blue ones. Light up red and green together and your cones signal just as they do for real yellow. This is called additive mixing. Paints work differently, because they absorb colours. Exactly how the brain creates the feeling of colour is still being studied.
Different Eyes, Different Colours
Some people have one kind of cone that works differently or is missing. This is called colour vision deficiency, or colour blindness. The most common kind makes red and green hard to tell apart. It affects about one in twelve boys and about one in two hundred girls. Most still see many colours, just fewer. Animals differ too. Dogs have two kinds of cones. Bees can see ultraviolet, light beyond violet that we cannot see, and many flowers have hidden ultraviolet patterns. This is for learning, not medical advice.
Now You Teach It
Let us put it all together. Sunlight is a mixture of all colours. Water and glass slow light down and bend it, and violet bends more than red, so raindrops spread sunlight into a rainbow, always opposite the Sun. Objects look coloured because of the light they reflect. In your eye, three kinds of cones send signals, and your brain turns the pattern into colour. Now try teaching a friend: why is a rainbow curved, and why does a leaf look green? If you can explain it simply, you truly understand it.