Optics
Rainbow science
A rainbow is more complex than it looks. Here is why they exist, why the colors are in that order, and the rare types you've probably never noticed.
Why rainbows exist
- Sunlight enters a raindrop.
- Different wavelengths refract by slightly different amounts.
- Light reflects off back of raindrop.
- Exits with slightly different angles for each color.
- Result: violet at inner edge (~40°), red at outer edge (~42°).
- Observer must be with sun BEHIND them.
- Rain must be IN FRONT.
- Sun angle below 42° or rainbow won't appear on ground.
The seven colors (or not)
- Traditional: red, orange, yellow, green, blue, indigo, violet.
- Newton chose 7 for religious reasons (7 planets, 7 days, 7 notes).
- Actual spectrum is continuous.
- Some cultures count 5 or 6 colors.
- Indigo is essentially indistinguishable from violet/blue.
- Newton probably included indigo for esthetic balance.
The double rainbow
- Second bow forms from double internal reflection.
- Colors reversed: red on inside, violet on outside.
- Appears at 51° instead of 42°.
- Dimmer than primary.
- Space between = Alexander's dark band.
- Requires clear rain and strong sunlight.
- Third and fourth rainbows exist but are on sun side, extremely rare to see.
Supernumerary bows
Faint colored bands just inside the primary bow.
- Caused by wave interference of light waves.
- Only visible when raindrops are very uniform in size.
- Multiple faint bows of alternating colors.
- Common in small droplets (~1 mm).
- Named for going "above and beyond" the primary bow.
- Also seen in mist and fog occasionally.
Rare rainbow types
Moonbow (Lunar rainbow)
Formed by moonlight. Faint, usually white to eye. Full spectrum revealed by long-exposure photo.
Fogbow
Formed in fog droplets. White or very pale colors due to small droplet size.
Reflection bow
From sunlight reflecting off water first.
Reflected bow
Seen from below the reflection, e.g., at a lake.
Twinned bow
Two primary bows that split — flat and round droplets mixed.
Red bow / Fire bow
Rare fire rainbow. Not actually a rainbow — circumhorizontal arc.
Circumzenithal arc
Above head. Ice crystals. Very colorful.
Sun dog / parhelion
Ice crystal optical. Adjacent to sun. Not a rainbow.
Halo
Ice crystal ring around sun/moon. Not a rainbow.
Iridescent cloud
Pastel colors from cloud thickness variations.
Polar stratospheric cloud
Iridescent polar clouds.
Photographing rainbows
- Sun behind you, rain in front.
- Polarizing filter enhances colors dramatically.
- Watch polarizer angle — rotates as you shift.
- ISO low for color depth.
- Wide-angle for full arc.
- Foreground element for context.
- Golden hour rainbows are richest.
- Full arc requires elevated position or aerial.
Rainbow lore
- Norse: Bifrost bridge between Asgard and Midgard.
- Irish: pot of gold at the end (which never exists, since bow moves with observer).
- Judeo-Christian: covenant after the Flood.
- Buddhist: symbol of enlightenment.
- Chinese: dragons and celestial serpents.
- Australian Aboriginal: Rainbow Serpent creator being.
- Modern: LGBTQ+ Pride, environmentalism, hope.
The optics of Alexander's band
The dark band between primary and secondary rainbows.
- Named for Alexander of Aphrodisias (2nd century AD).
- Region receives no light from either bow.
- Because primary and secondary refraction angles miss it.
- Physically real, not perceptual.
- Easy to see once you know to look.
When you'll see rainbows
- Late afternoon most common (sun low, storms building).
- Morning after front passage.
- Waterfalls and fountains — mini rainbows any time.
- After passing shower with sun re-emerging.
- Post-hurricane sunrise/sunset can produce dramatic rainbows.