Sunlight looks white, but it is actually all the colours mixed together. When light passes through a raindrop or a glass prism, it bends — and different colours bend by different amounts, so they spread apart. That's a rainbow: sunlight unwrapped.
Isaac Newton was the first person to prove this, in 1666. He let sunlight through a prism and saw all the colours appear. Then he put a second prism the other way — and all the colours merged back into white. He called it the spectrum.
Your phone or computer screen uses three colours of light — Red, Green, Blue — to make every colour you've ever seen on a screen. Drag the sliders and watch:
The rainbow we see is only a tiny slice of all the light that exists. Light comes in a huge range — called the electromagnetic spectrum. Most of it is invisible to human eyes:
Light travels at 300,000 kilometres per second. Nothing in the universe can go faster. At that speed, light circles the entire Earth 7.5 times every single second.
When you look at the Sun, you're seeing it 8 minutes ago — that's how long its light takes to reach us. When you look at distant stars, you're looking millions of years into the past. Every star you see at night might not even exist anymore.
The universe is so big that we measure distances in light-years — how far light travels in a year: 9.5 trillion kilometres.