Stand with the Sun behind you, look at a rain shower in front, and if you're lucky you'll see one of nature's most beautiful phenomena: a rainbow arching across the sky. This colorful arc has fascinated humans for as long as there have been humans — appearing in mythology from Norse to Greek to biblical. But the actual physics of how rainbows form is just as fascinating as the myths.
The Short Answer
Rainbows form when sunlight enters raindrops, refracts (bends) as it enters the water, reflects off the back surface of the droplet, and refracts again as it exits. Because different colors of light bend by different amounts (red bends less, violet bends more), the light separates into its component colors as it exits. The cumulative effect of millions of droplets creates the visible rainbow arc.
The Setup You Need
To see a rainbow you need three conditions:
- The Sun must be behind you.
- Rain (or water mist) must be in front of you.
- The Sun must be relatively low in the sky — within about 42° of the horizon.
That's why rainbows typically appear in the morning or late afternoon. Around noon (in the tropics or summer mid-latitudes), the Sun is too high — its rainbow would be below the horizon. You can sometimes create your own rainbow by spraying a garden hose into the air with the Sun behind you.
What Happens Inside a Raindrop
Each individual raindrop acts as a tiny prism:
- Sunlight enters the front surface of the drop. The light slows down as it enters the water and bends slightly — this is "refraction."
- Light hits the back of the drop and bounces back. This is "internal reflection."
- Light exits the front surface of the drop, bending again as it leaves.
The key fact: different wavelengths of light bend by slightly different amounts when refracting. Red light bends the least; violet light bends the most. This separation of colors is called "dispersion." Inside a raindrop, the dispersion is amplified by the double refraction (entering and exiting). The result: white sunlight enters the drop but exits as a spread of separated colors.
Why the Rainbow Is an Arc
The geometry of the refraction and reflection means that light exits each raindrop at a specific angle relative to the incoming sunlight. For red light, the angle is 42°. For violet light, it's 40°. Other colors fall in between.
So you see red light from drops at exactly 42° from the "antisolar point" (the point directly opposite the Sun from your perspective). And you see violet light from drops at 40°. The drops form a circle around the antisolar point — that's why rainbows are circular. You usually only see the upper half of this circle because the ground gets in the way. From an airplane, you can see a complete circular rainbow.
The Order of Colors
The standard rainbow shows seven colors in a specific order (from outside to inside):
- Red
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet
The mnemonic "ROYGBIV" helps remember the order. Isaac Newton actually divided the spectrum into seven colors specifically to match the seven notes of the musical scale — there's nothing inherent about seven; the spectrum is actually a continuous gradient. Some modern accounts merge indigo into blue, listing six colors (ROYGBV).
The Secondary Rainbow
Sometimes you can see a second, fainter rainbow outside the primary rainbow. This "secondary rainbow" forms when light reflects twice inside the raindrop instead of once. Because of the extra reflection, the secondary rainbow appears about 51° from the antisolar point, and the colors are reversed — red on the inside, violet on the outside.
Between the primary and secondary rainbows is a darker band of sky called "Alexander's band," named after the Greek philosopher Alexander of Aphrodisias who described it in 200 CE. This band is darker because no light reaches your eye from raindrops in that angle.
Higher-Order Rainbows
Tertiary and quaternary rainbows exist but are extremely faint. They're caused by three or four internal reflections. The tertiary rainbow is centered around the Sun (not the antisolar point) and is generally invisible because the Sun is in the way. Higher-order rainbows have been photographed but are vanishingly rare in nature.
Why Each Person Sees a Different Rainbow
Strictly speaking, no two people ever see the exact same rainbow. The rainbow you see depends on your specific viewing angle from each raindrop. The person next to you is seeing slightly different drops contributing to their rainbow. This makes rainbows uniquely personal — they're an interaction between specific drops and specific eyes.
This also means you can never reach the end of a rainbow. As you move, the rainbow moves with you, because the geometry depends on your position. The "pot of gold at the end of the rainbow" is, geometrically, impossible to reach.
Moonbows
When the Moon is bright (especially full moon) and conditions are right, you can see a "lunar rainbow" or "moonbow." These rainbows form the same way as solar rainbows but with moonlight instead of sunlight. Because moonlight is much dimmer, moonbows appear pale and white to human eyes — our color vision doesn't function well at low light levels. But cameras with long exposures can capture the full color of a moonbow.
Other Rainbow-Like Phenomena
Several optical phenomena are related to rainbows:
- Fogbows: White or pale rainbows formed in fog. The fog droplets are too small to disperse colors well.
- Halos: Caused by ice crystals in cirrus clouds, not water drops. They form circles around the Sun or Moon.
- Sun dogs (parhelia): Bright spots on either side of the Sun, also from ice crystals.
- Glory: A small colored ring around your shadow, often seen from airplanes — caused by water droplets in clouds.
- Aurora: Not rainbow-related, but another colorful sky phenomenon caused by solar particles.
Rainbows in Mythology
Almost every culture has rainbow mythology:
- Greek: Iris was the goddess of the rainbow, a messenger of the gods.
- Norse: Bifröst was the rainbow bridge connecting Earth to Asgard, home of the gods.
- Biblical: God's covenant with Noah after the flood was symbolized by a rainbow.
- Hindu: The rainbow is Indra's bow.
- Irish: A pot of gold lies at the end of a rainbow, guarded by leprechauns.
- Australian Aboriginal: The Rainbow Serpent is a major creation being in many traditions.
Newton's Investigation
Isaac Newton conducted famous experiments with prisms in the 1660s and 1670s that established white light is composed of all the spectrum colors. He showed that a prism breaks white light into colors (a "spectrum") and that a second prism can recombine them back into white light. This was foundational to understanding rainbows. Newton's 1704 book "Opticks" laid out the physics that explained rainbow formation.
How Drop Size Affects Rainbows
The size of the water drops in a rain shower affects how vivid the rainbow appears. Large drops (greater than 1 mm) produce sharp, vivid rainbows with well-defined colors. Smaller drops produce paler, broader rainbows with less color separation. When drops are very small (under 0.05 mm), the rainbow loses most of its color and appears as a white "fogbow." This is why fog and mist often produce whitish rainbows rather than colorful ones. Heavy thunderstorms with large raindrops typically produce the most spectacular rainbows.
The Supernumerary Bow
Sometimes, just inside the primary rainbow, you can see faint additional pastel bands of pink, purple, and green. These are "supernumerary bows" — extra rainbows formed by light wave interference rather than just refraction. They appear only when raindrops are uniformly small (around 0.5 mm). Supernumerary bows were a puzzle for early physicists because Newton's simple geometric optics couldn't explain them. Thomas Young's wave theory of light in the early 1800s finally provided the explanation, helping confirm light's wave nature.
Key Facts
- Rainbows form via refraction, reflection, and dispersion of light in water droplets.
- The primary rainbow appears 42° from the antisolar point.
- The secondary rainbow appears 51° from the antisolar point with reversed colors.
- Rainbows are technically full circles — you only see the upper half due to ground blocking the lower half.
- You can't reach the end of a rainbow because the geometry moves with you.
Fun Facts
- The mysterious 1964 abandoned lifeboat has never been explained.
- Bouvet is one of the most prized targets for ham radio DXpeditions.
- The island has a small dormant volcano (Olavtoppen, 780 m).
- Wind speeds regularly exceed 100 km/h.
- The nearest inhabited place is Tristan da Cunha, 2,260 km away.
The Bottom Line
Bouvet Island is the most remote island on Earth — a glacier-covered volcanic island in the South Atlantic, 1,700 km from any other land. Nobody lives there; nobody can easily visit. The island represents the geographic extreme of isolation and serves as a quiet monument to how vast our planet really is. For all of human civilization's reach, Bouvet remains essentially untouched.