Light Comes From Sources Like the Sun (How Rainbows Form)
Discover where light comes from, why white sunlight hides many colours, and how raindrops paint a rainbow.
Where light comes from
You already know that we need light to see. Imagine sitting in a room at night when the power goes out. Total darkness. You cannot see your hand or your toys. Then someone strikes a match, and everything appears. The match did something special: it made light. Things that make their own light are called light sources, or luminous objects. The Sun, a candle flame, a torch and a firefly are all light sources. Your hand and your toys are not. We can only see them when light lands on them.
Why hot things glow
But why does a flame give off light? Here is a clue. Watch a piece of iron in a blacksmith's fire. As it heats up, it glows dull red, then orange, then yellow, and when it is hottest of all, almost white. Anything that gets hot enough shines. Scientists call this incandescence, which means giving off light because something is very hot. Not every light source works this way. A firefly and an LED bulb make light while staying cool. They use chemistry or electricity instead of heat.
The Sun, our giant lamp
Our biggest light source is the Sun. It is not burning like a campfire. It is a gigantic ball of glowing gas, so big that about one million Earths could fit inside it. Deep in its centre, called the core, hydrogen is squeezed so hard that its tiny centres join together to make helium. This is called nuclear fusion, and it releases huge amounts of energy. The screen shows a simplified summary. That energy slowly works its way outward, and the Sun's surface glows white hot at about five and a half thousand degrees Celsius.
Light's long fast journey
Sunlight then travels through empty space to reach us. Light is the fastest thing there is. In empty space it moves about three hundred thousand kilometres every second. That is fast enough to go around the Earth seven and a half times in one second! Even so, the Sun is so far away that its light takes about eight minutes to reach you. Light also travels in straight lines. That is why shadows form. When your body blocks the straight path of the light, a dark patch appears behind you.
How we see things
So how do you see a book that makes no light of its own? Sunlight or lamplight hits the book and bounces off, a bit like a ball bouncing off a wall. This bouncing is called reflection. Some of the bounced light enters your eye. It passes through the clear front of the eye and the dark hole called the pupil, then reaches the back of the eye. There, special cells send signals to your brain, and your brain builds the picture. Even the Moon has no light of its own. Moonlight is reflected sunlight.
Hidden colours in sunlight
Sunlight looks plain and white. But in about sixteen sixty six, a young scientist named Isaac Newton tried an experiment. He darkened his room and let a thin beam of sunlight shine through a small hole. Then he passed the beam through a prism, which is a triangle-shaped block of glass. Out came a band of colours: red, orange, yellow, green, blue and violet. This band of colours is called a spectrum. Then Newton used a second prism to bring the colours back together, and they made white light again.
Colours are different waves
Why does each colour behave a little differently? Scientists describe light as a wave, a moving ripple of electric and magnetic energy, a bit like ripples spreading across a pond. The distance from one wave top to the next is called the wavelength. Red has the longest wavelength we can see, about seven hundred nanometres. Violet has the shortest, about four hundred. A nanometre is one billionth of a metre, far thinner than a hair. The wave is one useful picture. Light also behaves like tiny packets of energy called photons.
Light bends: refraction
Put a pencil in a glass of water and look from the side. The pencil seems broken at the water's surface! This happens because light travels more slowly in water and glass than in air. In water it goes about three quarters as fast. When light hits the surface at a slant, this change of speed makes it change direction. Picture a toy car rolling at an angle from a smooth floor onto a rug. The wheel that touches the rug first slows down, so the car swerves. This bending of light is called refraction.
Each colour bends differently
Now we can explain Newton's prism. Inside glass, violet light slows down a little more than red light. So when white light enters a prism, violet bends the most and red bends the least. The other colours bend by amounts in between. The colours were all mixed together in the white light, and now they fan out and separate. This spreading out is called dispersion. Think of runners who start a race side by side but cross muddy ground at slightly different speeds, so they spread out.
Raindrops as tiny prisms
A rainbow is the same trick, done by millions of raindrops. Falling raindrops are nearly round balls of water, not teardrop shapes. Let's follow one ray of sunlight. First, it enters the drop and bends, and the colours begin to separate. Second, it hits the back of the drop and reflects, like light off a mirror. Third, it leaves the front of the drop and bends again, which spreads the colours even more. Each drop sends coloured light back towards the side the Sun is on, angled slightly downward.
Where to find a rainbow
To see a rainbow, stand with the Sun behind you and rain in front of you. Red light reaches your eye from drops about forty two degrees away from the point directly opposite the Sun. Violet comes from about forty degrees, so red is on the outside of the bow and violet is on the inside. All the drops at that angle form a circle, so a rainbow is really a circle. From the ground, the Earth hides the bottom part. That is why rainbows appear when the Sun is low in the sky.
Double rainbows
Sometimes you see a second, fainter rainbow outside the first one. For this one, the light reflects twice inside each drop before it leaves, and it reaches you at about fifty one degrees. The extra bounce flips the order of the colours, so this rainbow has red on the inside and violet on the outside. Between the two bows, the sky looks darker. This is called Alexander's dark band, because fewer drops send light to your eye from there. Newton named seven colours, including indigo, but really the colours blend smoothly into each other.
Common mistake
Many people think the prism or the raindrop adds the colours, like paint. It does not. The colours were already hidden in white sunlight, and the water only sorts them out. Newton showed this: when he sent only red light through a second prism, it stayed red. A second mistake is thinking a rainbow sits in one place. A rainbow is not an object. Each person sees light coming from different drops. So your friend beside you sees a slightly different rainbow, and nobody can ever reach the end of one.
Your turn to explain
Let's put it all together. Light sources like the Sun make their own light, often because they are very hot. Light travels very fast and in straight lines. White sunlight is a mix of colours. Raindrops bend the light, reflect it, then bend it again, and each colour bends by a different amount, so the colours spread into a bow. Some questions are still deep. Light acts like both a wave and a particle, and scientists still find that strange. Now explain to a friend why rainbows appear only when the Sun is behind you.