The Rainbow Fish of Umiam Lake
Fish that shimmer — the optics of iridescence.
The Story
The Lake Above the Clouds
High in the Khasi Hills of Meghalaya, cradled between pine-covered ridges, lies Umiam Lake. The people of the hills call it the lake above the clouds, because on misty mornings the water floats in a white sea of fog as if it has left the earth entirely.
Many fish live in Umiam — silver fish, brown fish, quick little fish that dart between the rocks. But there is one fish that is different from all the others. The elders call her Ka Ri Bneng — the Sky Fish — and they say her scales are made of captured rainbows.
Nobody agrees on what colour she is. That is what makes her magical.
The Girl Who Wanted to See
A Khasi girl named Daphisha lived in the village above the lake. She was twelve, sharp-eyed, and stubborn — the kind of girl who would argue with a thunderstorm and win.
One morning, her grandmother told her about Ka Ri Bneng. "When a happy person looks at the fish, her scales shine gold. When a sad person looks, she turns blue. When an angry person looks, she burns red. The fish shows you what you're feeling, even when you don't know yourself."
"That's impossible," said Daphisha. "A fish can't change colour because of someone else's mood."
"Then go see for yourself," said her grandmother, smiling.
Three Visits
Daphisha went to the lake that very afternoon. She was excited — almost giddy — because she loved a mystery. She sat on the mossy bank, looked into the clear water, and waited.
After twenty minutes, a large fish drifted out from behind a submerged rock. Her scales caught the light and shimmered a brilliant, blazing gold. Daphisha gasped. The fish was the most beautiful thing she had ever seen.
Gold, she thought. Grandmother said gold means happy. I suppose I am happy right now.
She came back the next day. This time, she had fought with her best friend that morning. The argument was about something stupid — whose turn it was to carry the water — but it had left a sour knot in Daphisha's chest. She sat by the lake and waited.
Ka Ri Bneng appeared again. But today, her scales were a deep, bruised violet — the colour of a thundercloud before it breaks. Daphisha stared. The fish hadn't changed shape or size. Only the colour was different.
On the third day, Daphisha came to the lake after her little brother had fallen from a tree and broken his arm. She was frightened and shaken, her heart racing with worry. When Ka Ri Bneng appeared, her scales pulsed a pale, trembling white — the colour of lightning, the colour of fear.
The Fish's Secret
Daphisha ran home to her grandmother. "The fish really does change colour!" she said. "She was gold, then violet, then white. How does she do that?"
Her grandmother poured her a cup of sha lamet — the smoky Khasi tea — and sat down slowly. "The fish doesn't change, my heart. The fish has every colour in her scales, all the time. What changes is which colour your eyes choose to see."
Daphisha frowned. "My eyes don't choose."
"Oh, but they do. When you are happy, your eyes are drawn to warmth and gold. When you are hurt, they seek out bruise-colours. When you are afraid, they find the cold, white flicker. The fish is a mirror, Daphisha. She shows you yourself."
What Daphisha Learned
Daphisha went back to the lake many more times that year. She learned to read her own moods by watching the fish — and slowly, she learned something even more important. She could change the colour she saw. If she arrived angry and sat quietly, breathing the pine air, watching the clouds drift below her, the red would fade to amber, then to gold.
She couldn't control the fish. But she could steer herself.
Years later, when Daphisha became a teacher, she would take her students to Umiam Lake on the first day of every school year. "Look into the water," she would say. "Tell me what colour you see. Then we'll talk about why."
And somewhere below the surface, Ka Ri Bneng would drift past — her scales shimmering with every colour at once, waiting to show each child exactly what they needed to see.
The end.
Before you start
Try It Yourself
Choose your level. Everyone starts with the story — the code gets deeper as you go.
Story Progress
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Here is a taste of what Level 1 looks like for this lesson:
import numpy as np
import matplotlib.pyplot as plt
# Thin-film interference: which color wins?
thickness = 120 # nm (guanine platelet thickness)
n_guanine = 1.83 # refractive index of guanine
wavelengths = np.linspace(380, 700, 300) # visible light, nm
# Constructive interference when 2nt = m * wavelength
path_diff = 2 * n_guanine * thickness # ~439 nm
# Intensity peaks where path difference = whole number of wavelengths
intensity = np.cos(np.pi * path_diff / wavelengths)**2
plt.figure(figsize=(10, 5))
plt.fill_between(wavelengths, intensity, alpha=0.3, color='dodgerblue')
plt.plot(wavelengths, intensity, linewidth=2, color='dodgerblue')
plt.xlabel("Wavelength (nm)")
plt.ylabel("Reflected intensity")
plt.title(f"Fish scale iridescence: ${thickness} nm guanine platelet")
plt.axvline(path_diff, color='red', linestyle='--', label=f'Peak at ${path_diff:.0f} nm')
plt.legend()
plt.show() # Which color is brightest? What happens if the platelet is thicker?This is just the first of 6 coding exercises in Level 1. By Level 4, you will build: Investigate Iridescence in Everyday Objects.
By Level 4, enrolled students build: Investigate Iridescence in Everyday Objects
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Level 0 is always free. Coding levels (1-4) are part of our 12-Month Curriculum.