It's the rainy season, and you step outside for a stroll. You glance up and there it is — a beautiful arc of seven colours — red, orange, yellow, green, blue, indigo, violet — stretched across the grey sky. A rainbow. It still stops you for a second every time. But do you know why it happens, or how? Here's a twist to sweeten the question: you don't even need rain for a rainbow. Buckle up — this one's a good one.
What's Really Going On Up There
Sunlight — what we call white light — is actually made up of many colours, including all the ones you see in a rainbow. In simple terms, a rainbow forms when sunlight gets scattered by water droplets. But let's go a little deeper, because that's where it gets interesting.
When sunlight meets a water droplet, two things happen almost at once: refraction, where the light bends as it moves from air into water, and total internal reflection, where it bounces off the back of the droplet and exits. As the light leaves the droplet, it bends again — and the colours split apart, becoming visible to the naked eye. That split is the rainbow.
It's the same behaviour you see when you shine a white light through a prism and watch a spectrum of colours appear on the other side. Same physics — it just happens to be playing out inside a raindrop.
Here's where it gets even more clever. Each colour bends at a slightly different angle depending on its wavelength. Red, which has the longest wavelength we can see, appears higher in the sky than violet, which has the shortest. Different droplets emit different colours based on the angle at which light strikes them, and all that scattered light comes together to form the arc you see. It's a huge collaboration of millions of tiny raindrops.
How to Actually See One
There are a few conditions that need to line up for a rainbow to appear in front of you:
- The sun needs to be behind you.
- The lower the sun sits in the sky, the wider the arc will be.
- And, of course, there need to be water droplets in the air — rain, fog, or even a fountain works if you've got the first two conditions sorted.
What You're Not Seeing
Here's a little extra learning for the curious mind. Light doesn't contain just seven colours — there are more than that. Our eyes can only perceive a portion of them, and indigo in particular is hard for most of us to make out, which is why it tends to blur into a messy mix between blue and violet. The "seven colours of the rainbow" is partly a simplification that stuck.
And that calm half-arc you see from the ground? That's because you're standing on it. If you ever spot a rainbow from an airplane, you can see a full circle — a far more dramatic sight than the semicircle we're used to.
One more question you might ask: why do the droplets matter at all? Why doesn't the air itself do this? Because water is a denser medium than air. Think of how your movement changes in a crowded passage versus an open one — the medium makes all the difference in how the light behaves.
Curiosity satisfied? There's always another question — like why the clouds turn dark before it rains. That one can wait for next time.
And if the sky's colours intrigue you, you'll enjoy the story behind why the sky is blue and sunsets are red.