
The physics of why the sky turns golden over the Brahmaputra.
The Sky Painter
Long before humans looked up and gave the sky a name, there was a spirit whose only job was to paint the sunset. Her name was Rongili, which means the colourful one, and she lived on a cloud so high that even eagles couldn't reach it.
Every evening, Rongili dipped her brushes into pots of colour and swept them across the western sky. She had pots of red for the deserts, pink for the oceans, purple for the mountains, and gold for the plains. Each place on Earth got its own sunset, mixed and painted fresh every single day.
Rongili loved her work. But she had a secret favourite.
The Favourite Place
Of all the lands Rongili painted sunsets for, she loved Assam the most. She loved the way the Brahmaputra caught her colours and doubled them, so the sky and the river burned together. She loved the tea gardens, where her light turned the green leaves to copper. She loved the paddy fields, where her sunset reflected in a thousand tiny mirrors of standing water.
"Assam deserves the best sunset," Rongili said every evening. And every evening, she mixed a little extra colour — a deeper red, a brighter gold, a softer pink — just for the skies above the Brahmaputra valley.
The other spirits noticed. "You give Assam more colour than anywhere else," said the wind spirit. "That's not fair."
"I can't help it," said Rongili. "It's my favourite place. The river is wide enough to hold the whole sky in it. Where else can a sunset see its own reflection so clearly?"
The Last Pot of Orange
One autumn, something terrible happened. Rongili's paint pots began to run dry. She had been painting sunsets for ten thousand years, and the colours were finally running out.
First the purple ran dry. Then the pink. Then the gold. One by one, the colours dwindled until Rongili was left with a single pot — her orange. A deep, warm, glowing orange, the colour of embers in a winter fire, of marigolds at a festival, of a ripe komola from an Assam garden.
One pot. Enough for one last sunset.
Rongili looked down at the Earth. She could paint this final sunset anywhere — over the Sahara, over the Pacific, over the Himalayas. Any place would be beautiful. But she already knew where it would go.
"Assam," she whispered. "Always Assam."
The Greatest Sunset
That evening, Rongili dipped her largest brush into the orange pot and painted the sky above the Brahmaputra with everything she had. She didn't hold back. She didn't save a drop for tomorrow. She swept the orange across the clouds in great, generous strokes — tangerine near the horizon, amber higher up, apricot at the edges, and a deep, burning vermillion at the centre where the sun touched the river.
The entire valley glowed. The tea gardens turned to fields of flame. The river became a ribbon of molten copper. The white egrets flying home looked like sparks rising from a bonfire. Even the rhinos in Kaziranga stopped eating and turned their heads west to watch.
Children ran out of their houses. Grandmothers stood in their doorways. Fishermen paused mid-cast, their nets hanging still, their eyes wide.
It was the most beautiful sunset anyone had ever seen.
The Promise That Stayed
When the last light faded, Rongili looked at her empty pot and smiled. "That was worth ten thousand years of painting," she said.
But something unexpected happened. The next evening, as the sun went down, the sky above Assam turned orange again — all by itself. Not because Rongili painted it, but because the colour had soaked so deeply into the clouds and the river and the air that it had become part of the place. Rongili's last gift had stained the sky forever.
And so, to this day, the sunsets over Assam are orange. Not pink like the coast, not purple like the mountains, not gold like the plains — but a deep, warm, glowing orange that fills the whole valley and turns the Brahmaputra into a river of fire.
The people of Assam say that if you watch the sunset quietly, you can feel Rongili's presence — the sky-painter who loved their land so much she gave it her very last colour.
The end.
Choose your level. Everyone starts with the story — the code gets deeper as you go.
Here is a taste of what Level 1 looks like for this lesson:
import numpy as np
import matplotlib.pyplot as plt
# The Rayleigh scattering formula
# Blue light scatters much more than red!
wavelengths = np.linspace(380, 700, 100) # nm
scattering = 1 / wavelengths**4 # Rayleigh's law
scattering = scattering / scattering.max()
plt.figure(figsize=(10, 5))
plt.fill_between(wavelengths, scattering, alpha=0.3)
plt.plot(wavelengths, scattering, linewidth=2)
plt.xlabel("Wavelength (nm)")
plt.ylabel("Scattering intensity")
plt.title("Why Blue Scatters More Than Red")
plt.show() # What does this plot tell you?This is just the first of 6 coding exercises in Level 1. By Level 4, you will build: Build a Sunset Color Simulator.
By Level 4, enrolled students build: Build a Sunset Color Simulator
Free
Level 0: Listener
Stories, science concepts, diagrams, quizzes. No coding.
You are here
Level 0 is always free. Coding levels (1-4) are part of our 12-Month Curriculum.
The physics of why the sky turns golden over the Brahmaputra.
The big idea: "Why Assam's Sunsets Are Orange" teaches us about Atmospheric Optics & Scattering — and you don't need to write a single line of code to understand it.
Sunlight looks white, or maybe pale yellow. But is it really one thing? Here is an experiment you can try: hold a glass of water up to sunlight at an angle, so the light passes through and hits a white wall. You will see a faint rainbow on the wall — red, orange, yellow, green, blue, violet. The white light split into colours.
A glass prism does this more dramatically. Isaac Newton was the first to show (in 1666) that white light is actually a mixture of all colours. Each colour has a different wavelength — think of it as the spacing between wave crests. Red has the longest waves, violet the shortest, and all the other colours fall in between:
When all these wavelengths arrive at your eyes together, your brain sees "white." When they are separated, you see the individual colours. A rainbow is sunlight being separated by raindrops. The colours on a CD surface are sunlight being separated by tiny grooves. The sunset you see from the Brahmaputra is sunlight being filtered by the atmosphere. Same principle every time.
Check yourself: If you removed all the blue light from sunlight, what colour would the remaining light look? (Think about it — you will find the answer in the sunset section below.)
Key idea: White light is a mixture of all visible colours. Each colour has a different wavelength — red is the longest, violet is the shortest. When they separate, you see a rainbow.
Imagine tossing a tennis ball into a field of tall grass. The ball bounces off the grass stalks in random directions — it gets scattered. Now imagine tossing a football into the same grass. The football is too big to be deflected by thin grass stalks, so it ploughs straight through. The tennis ball gets scattered; the football does not.
Something similar happens to light in the atmosphere. Air is made of tiny molecules (mostly nitrogen and oxygen). When sunlight hits these molecules, the short-wavelength colours (blue and violet) are small enough to bounce off in all directions — they get scattered. The long-wavelength colours (red and orange) are too "big" to be scattered much, so they pass straight through. This is called Rayleigh scattering.
The effect is dramatic: blue light scatters about 5–6 times more than red light. So when you look at any part of the sky away from the sun, you are seeing blue light that was scattered sideways toward your eyes. The sky is literally glowing with scattered blue light.
But wait — violet has an even shorter wavelength than blue, so it should scatter even more. Why isn’t the sky violet? Two reasons: the sun produces less violet light to begin with, and your eyes are much less sensitive to violet. So even though violet scatters the most, your eyes pick up more blue. The sky looks blue, not violet, because of your biology as much as the physics.
Try this: On a clear day, look at the sky near the horizon and then straight up. The horizon sky looks paler — almost white. Why? Because the light reaching you from the horizon has been scattered many times through more atmosphere, mixing all colours together back toward white.
Key idea: Blue light scatters 5–6 times more than red light when it hits air molecules. You see this scattered blue light everywhere you look in the sky. Violet scatters even more, but your eyes are less sensitive to it.
Access all 130+ lessons, quizzes, interactive tools, and offline activities
At noon, sunlight travels straight down through about 10–20 km of atmosphere to reach you. At sunset, the sun is near the horizon, and its light has t...
Expected Output — Your Sunset Simulator Should Produce This
Each strip shows sky colour from zenith (top) to horizon (bottom) at different sun angles