Light Travels in Straight Lines Called Rays (How Rainbows Form)
Light zooms in straight lines, bounces, bends and splits into colours, and raindrops use all three tricks to paint a rainbow.
Light You Can See
Last time we learned that light is a kind of energy our eyes can detect. Now let us watch where it goes. Imagine a dusty room on a sunny morning. A beam of sunlight pours through a gap in the curtains. You can see the beam because tiny bits of dust bounce some light into your eyes. Look at its edges. They are sharp and straight, like a ruler. The beam does not wiggle or curve around the room. That is our first clue.
Testing Straight Lines
Here is a test you can do. Take three cards and poke a small hole in the middle of each. Line them up and shine a torch through. You see the light at the far end only when all three holes sit in a perfectly straight line. Move one card a little sideways and the light disappears. Light will not take a bent path. Scientists call a thin straight line of light a ray. A ray is a drawing tool, a simplified picture of the path light takes.
Shadows Prove It
Shadows give more proof. When something solid blocks the light, the rays that hit it stop, and the rays that miss it keep going straight past. Behind the object is a dark patch where no rays arrive. That is a shadow. Its shape matches the outline of the object, because light does not curl around corners to fill the gap. In the evening the sun is low, its rays arrive at a slant, and your shadow stretches long across the ground.
Fast and Bouncing
Light is the fastest thing there is. In empty space it travels about three hundred thousand kilometres every second. That is fast enough to go around the Earth about seven and a half times in one second. When a ray hits a shiny mirror, it bounces back. This is called reflection. The ray leaves the mirror at the same angle it arrived, like a ball bouncing off a wall. Before and after the bounce, it still travels in straight lines.
Bending at the Edge
Put a pencil in a glass of water. It looks broken at the surface! The light is doing something new. Light travels more slowly in water than in air, about three quarters as fast. When a ray crosses from one material into another at a slant, the change of speed makes it change direction. Imagine a toy car rolling at an angle from a smooth floor onto a carpet. The wheel that reaches the carpet first slows first, so the car turns. This bending is called refraction.
White Light Hides Colours
About three hundred and sixty years ago, Isaac Newton sent a thin beam of sunlight through a triangle of glass called a prism. Out came a band of colours: red, orange, yellow, green, blue and violet. Then he used a second prism to mix them back into white. So white light is not one colour. It is a mixture of all the colours together. The band of colours is called a spectrum. Light also behaves like a wave, and each colour has a different wavelength.
Each Colour Bends Differently
Why does a prism spread the colours apart? Because the colours do not all slow down by the same amount in glass. Violet light slows a little more than red light, so violet bends the most and red bends the least. Each colour leaves the prism in a slightly different direction, and the mixture fans out like a hand of cards. Spreading white light into colours this way is called dispersion. Water does exactly the same thing, and that is the secret of the rainbow.
Raindrops as Tiny Prisms
After a shower, the air is full of round raindrops. Follow one sunbeam into one drop. First it bends as it enters the water, and the colours begin to separate. Then it hits the back of the drop, and some of the light reflects, as if from a curved mirror. Then it bends again as it leaves through the front, spreading the colours even more. The light comes back out toward the side where the sun is, split into colours. Each drop is a tiny prism and mirror together.
The Magic Angle
The light coming back out of the drops is brightest at one special angle. Imagine a line from the sun, through the back of your head, to the shadow of your head. Red light reaches you strongly from drops about forty two degrees away from that line. Violet comes from drops about forty degrees away. So each drop sends you mainly one colour, depending on where it sits. Millions of drops together paint the bands, with red on the outside and violet on the inside.
Where to Find One
Stand with the sun behind you and rain in front of you. The point straight opposite the sun, where the shadow of your head points, is the centre of the rainbow. All the drops forty two degrees from that point make a circle of colour. Usually the ground hides the lower part, so you see an arch. From an aeroplane, people sometimes see a full circle. When the sun is higher than forty two degrees, the bow sinks below the horizon, so rainbows appear mostly in the morning or late afternoon.
Double Rainbows
Sometimes a fainter second rainbow appears outside the first. Here the light reflects twice inside each drop before it escapes. Every reflection lets some light leak away, so the second bow is dimmer. It sits about fifty one degrees from the centre, and its colours are flipped, with red on the inside. Between the two bows the sky looks darker, because few drops send light to you from those angles. This is called Alexander's dark band, after a Greek thinker who described it about eighteen hundred years ago.
Common mistake
Many people think a rainbow is a real object hanging in one place, with an end you could walk to. It is not. A rainbow is an angle, not a thing. It depends on where your eyes are. When you move, different drops line up at forty two degrees, so your rainbow moves with you. A friend standing beside you gets light from different drops, so each person sees their own rainbow. Another mistake is thinking it has exactly seven colours. The colours blend smoothly; seven is just a naming habit.
Where Rays Stop Working
The ray picture is a model, a simplified drawing that works brilliantly for shadows, mirrors and rainbows. But it is not the whole truth. Light also behaves like waves, and waves spread out a little at edges, which rays cannot explain. In fog, the drops are so tiny that this spreading blurs the colours together into a pale, almost white fogbow. Faint extra stripes just inside some rainbows also need the wave idea. And layers of hot and cool air can gently curve light, making mirages.
Your Turn to Teach
Let us gather it up. Light travels in straight lines called rays, which is why shadows have sharp edges. It reflects off mirrors, and it bends, or refracts, when it changes speed. White light is a mixture of colours, and each colour bends a slightly different amount. Raindrops bend, reflect and bend the sunlight, sending each colour back at its own angle. Now try it yourself. Explain to a friend why the sun must be behind you to see a rainbow, and why their rainbow is not quite yours.