Why is the
sky blue?
Grown-ups say "because of the air" and walk off. Silly. Here's what's really happening — and you'll find it yourself by poking things.
Surprise! Sunlight is a trick.
That white beam looks like one colour. It isn't. It's every colour at once, stacked up so tight your eye can't tell them apart. Let's pull them out and look.
The Rainbow Secret
Slide the glass prism into the sunbeam and hold on.
👆 Drag the slider. Push the prism right into the light.
Now meet the seven. Tap one.
Each colour is a wave, and every wave is a different size. That turns out to be the whole secret.
Pick a colour above to see how big its wave is. 👆
The Bounce Chamber
This box is a teeny piece of air. Those wobbling dots are bits of gas — invisible, but they're in your lungs right now. Pick a colour of light and ZAP it through. Count the bounces.
Your results
Try one more thing: slide AMOUNT OF AIR way up and zap white-ish light. More air = more bouncing = paler, milkier sky. That's why the sky near the horizon looks washed out.
Wait. Three good questions.
You're asking the exact things clever kids ask. Tap each card.
If violet bounces most… why isn't the sky violet?
Why is the whole sky blue, not just one spot?
Why is the air right round the Sun pale white?
The Sunset Machine
Pull the Sun down to the horizon and watch the light have to fight its way through the air to reach your eyes.
At noon the light drops almost straight down — a short trip. The blue leaps out sideways all over the sky, so you see blue everywhere. 🔵
The bold bit: when the Sun sits low, its light has to scrape through a huge slab of air to reach you. Every bit of blue gets thrown out of the beam long before it arrives. What's left over? The stubborn reds and oranges. They're the only ones that make it to your eyes — and they paint the clouds too.
Switch the air OFF
Same Sun. Same time of day. Only one thing removed.
Astronauts on the Moon stand in bright sunlight and the sky above them is black, with stars shining. No air → nothing to bounce the blue → no blue sky. That's the proof that the blue comes from the air, not from the Sun.
Show what you know
Make a sky inside a cup
You can do the exact same trick the atmosphere does — at the kitchen table. (Ask a grown-up to help with the dark.)
Stir in half a teaspoon of milk.
Not a splash — tiny. The milk bits are going to play the part of the air bits.
Shine the torch through the cup.
Hold the paper behind it. The light landing on the paper will look yellow, orange or even red — that's your sunset, made by you.
Now look at the cup from the side.
The water glows a faint milky blue. That's the same blue light being thrown sideways at your eyes — the sky's trick, shrunk down.
Push it further.
More milk → bluer sides, redder far side. Too much → everything goes cloudy white and the trick dies. (You just made fog!)
⚠️ Don't drink it afterwards — torch-water-milk is gross, and cups aren't clean.
Your turn to explain it
Real scientists have to say it out loud. Finish the thought in your own words — there's no wrong answer here.
For the grown-up beside them
The short version, honestly — plus the things this lab quietly skipped.
What's actually happening
- Raleigh— sorry, Rayleigh scattering: particles much smaller than the wavelength of light scatter it with an intensity proportional to 1/λ⁴. That's why the sim uses ratios of roughly 1 : 2 : 6 : 9 for red : yellow : blue : violet.
- Sky colour is a mixture, not pure blue — the spectrum of daylight peaks in the green, yet we call it blue. Hue perception and the eye's cone response do the rest.
- Violet is also partly absorbed up high by ozone (the Chappuis bands), which nudges the colour toward blue.
- Paler horizon and milky skies are Mie scattering from larger aerosols (water droplets, dust, pollution) — nearly colour-blind, so it whitens. That's the milk-in-a-cup effect and the "amount of air" slider.
- Deep red sunsets get an extra push from ozone reddening and are boosted by dust and aerosols — which is why sunsets after a big wildfire or volcanic eruption are spectacular.
- The Sun itself would look white from space, and slightly green-ish peaked; sunsets tint the disc because the shortest wavelengths have been removed from the direct beam.
- Not covered here: why the sky isn't blue at night (obviously), why clouds are white (thick multiple scattering), and why Mars' sky is butterscotch (fine iron-dust aerosols scattering forward — Mie, not Rayleigh). Ask about Mars and you'll have a whole second lesson ready.
Questions to keep the ball rolling
- "If blue scatters the most, why are your eyes not blue when you look at someone?" (They're not lit from a thick slab of air.)
- "What colour is the sky on Mars? On Titan? On Pluto?"
- "Why do stars twinkle but planets don't?" (Great follow-on: turbulence, not scattering.)
- "Could you make a red sky at noon? How?" (More air! Or thicker atmosphere — which is what you tested with the milk.)