Atmospheric Optics
Rainbow, haloes, coronae, glories. Builds on Snell's Law & Refraction.
On a cold, clear night you notice a perfect ring of light around the Moon at exactly 22° from its center. Your friend says it’s caused by water droplets.
What actually causes the 22° lunar halo?
Ice crystals, not water drops! High-altitude cirrus clouds contain tiny hexagonal ice crystals. Moonlight refracts through the 60° prism faces of these crystals. The minimum deviation angle for ice is 22°, creating a bright ring at that exact angular distance. Inside the ring is dark (no light deflected by less than 22°). The ring has a slightly reddish inner edge because red light deviates less than blue.
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Watch light rays enter hexagonal ice crystals and refract through two faces. The minimum deviation angle creates the 22° halo. Toggle different atmospheric phenomena: rainbows (water drops), coronae (diffraction), and glories (backscattering).
δ_min = 2 arcsin(n sin(α/2)) − α
The 22° halo is exactly the minimum deviation angle for a 60° ice prism. The sky inside the ring is dark because no light can be deflected by LESS than the minimum deviation. This is the same physics as a glass prism, but with ice crystals in the sky.
The most common halo. Folklore says it predicts rain or snow within 24 hours — actually reasonably accurate, since cirrus clouds (which contain the ice crystals) often precede weather fronts.
Bright rainbow-tinged spots 22° to the left and right of the Sun. Caused by horizontally oriented plate-shaped ice crystals. Best seen when the Sun is low. Vikings may have used them for navigation!
The colorful rings around your airplane’s shadow on clouds. Each passenger’s glory is centered on their OWN head’s shadow (a personal optical phenomenon). Caused by complex wave effects in water droplets.
Faint extra color bands just inside the primary rainbow are caused by interference between rays that take slightly different paths through the raindrop. They’re more visible when droplets are small and uniform.
“The sky is an optics laboratory. Ice crystals make halos, water drops make rainbows, tiny particles make coronae. Every ring and arc follows from refraction, reflection, or diffraction — the same physics, written across the sky.”
Atmospheric Optics Lab
Explore the optics of atmospheric phenomena. Trace rays through ice crystals (halos), water drops (rainbows), and small particles (coronae). See how crystal orientation, drop size, and viewing geometry create the spectacular displays we see in the sky.
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Start with the 22° halo: send parallel light through a hexagonal ice crystal and find the minimum deviation angle. Rotate the crystal and see how the deviation changes but never goes below 22°. Switch to a water drop to trace rainbow rays — find the 42° minimum deviation for the primary bow. Try the corona mode with small droplets: adjust the droplet size and watch the ring diameter change (smaller drops = larger rings). Compare all three phenomena side by side.
Each atmospheric optical phenomenon uses different physics: halos use refraction through ice prisms, rainbows use refraction + total internal reflection in water drops, coronae use diffraction by small particles. The common thread is that minimum deviation angles create bright features at specific, predictable angles.