More Than Just Colour: The Physics of a Rainbow
A rainbow is one of nature's most instantly recognizable phenomena — and also one of the most precisely understood. Far from being mysterious, rainbows are the result of well-defined optical physics: the refraction, reflection, and dispersion of sunlight inside spherical water droplets suspended in the atmosphere.
Understanding how rainbows form won't make them any less beautiful. If anything, knowing the science makes each sighting feel even more remarkable.
The Three Key Processes: Refraction, Reflection, Dispersion
1. Refraction
When a ray of sunlight enters a raindrop, it slows down and bends — a process called refraction. Light travels more slowly in water than in air, and this speed change causes the light ray to change direction. Different wavelengths of light bend by slightly different amounts, which is what ultimately separates white sunlight into its component colours.
2. Internal Reflection
Once inside the droplet, the light hits the back surface and reflects internally, bouncing back the way it came (though at an angle). This internal reflection is what sends light back toward the observer rather than passing straight through the droplet.
3. Dispersion
As the reflected light exits the droplet through the front surface, it refracts a second time. Because each colour of light has a different wavelength, each exits at a slightly different angle. Red light exits at about 42°, while violet exits at about 40°. This 2° spread is enough to separate the colours into the familiar arc of the rainbow.
Why Rainbows Are Always Arcs
The arc shape isn't artistic licence — it's geometry. The rainbow always appears at a fixed angle (approximately 42° for red, 40° for violet) from the antisolar point, which is the point directly opposite the sun from the observer's perspective. Since this angle is constant, and the antisolar point is always directly in front of you, the visible portion of the rainbow forms a circular arc around that point.
From the ground, the horizon cuts this circle in half, so you see a semicircle at most. From an aircraft above the clouds, you can sometimes see a full circular rainbow — a breathtaking sight.
Double Rainbows Explained
A secondary rainbow — the fainter, wider arc sometimes visible outside the primary bow — forms when light undergoes two internal reflections inside the raindrop instead of one. This extra reflection does two things:
- It reverses the colour order: red appears on the inside of the secondary bow, violet on the outside (opposite to the primary).
- It makes the bow dimmer, since some light escapes with each reflection.
The region between the two bows, called Alexander's dark band, appears noticeably darker than the rest of the sky — a subtle but beautiful optical effect worth looking for.
Other Rainbow Variants Worth Knowing
- Moonbow (lunar rainbow): A rainbow produced by moonlight instead of sunlight. Appears white or very pale to the human eye because moonlight is too dim to activate colour vision strongly, but full colour is revealed in long-exposure photographs.
- Fogbow: Formed by tiny water droplets in fog. Because the droplets are so small, diffraction smears the colours together, producing a broad, nearly white arc with faint reddish and bluish edges.
- Red rainbow: Seen at sunrise or sunset when the sun is very low. Blue and green light is scattered away by the long atmospheric path, leaving only red and orange wavelengths to form the bow.
- Supernumerary rainbow: A series of faint, closely spaced arcs just inside the primary bow's violet edge, caused by wave interference between light paths through similarly sized droplets.
The Best Conditions for Rainbow Spotting
To see a rainbow, you need three things in alignment:
- Rain or water droplets in the air ahead of you
- Sunlight behind you — the sun must be shining on your back
- Low sun angle — the sun should be no more than about 42° above the horizon, meaning rainbows are most common in the morning or late afternoon
The most vivid rainbows occur with large, uniform raindrops and a clear sky behind the rain shower so the full arc is visible against a dark backdrop.