How Rainbows Form
It is like a giant, colorful fan painted across the sky as millions of airborne water droplets bend, bounce, and spread sunlight.
Definition A rainbow is an optical phenomenon created when sunlight enters countless tiny water droplets floating in the air, bending and reflecting the light into a spectrum of colors. The seemingly transparent sunlight actually contains every color from red to violet, which bend at different angles through the droplets before reaching our eyes.
The Three-Step Journey of Light Inside a Droplet
After it rains, countless microscopic water droplets hang suspended in the air. When sunlight hits these droplets from behind you, it slows down and bends at the boundary between air and water. This is called refraction.
Once inside the droplet, the light does not simply pass straight through. Instead, it hits the inner back wall of the drop and bounces back like a mirror. This is internal reflection. While some light escapes, the reflected light travels back toward the front of the droplet.
Finally, as the light exits the front of the droplet back into the air, it bends once more. By going through this three-step process of refraction, reflection, and refraction, the hidden colors of white light separate and become visible to our eyes.
Why Is Red Always on the Outside?
Sunlight appears white, but it is actually a mixture of every color, from red with long wavelengths to violet with short wavelengths. When light enters a water droplet, each color bends at a slightly different angle.
Short-wavelength violet light bends sharply, while long-wavelength red light bends more gently. This spreading of colors at different angles is called dispersion.
As light exits each droplet, red light leaves at an angle of about 42 degrees relative to incoming sunlight, while violet light leaves at about 40 degrees. Because higher droplets in the sky direct red light down into your eyes while lower droplets direct violet light, red always appears at the top and outer edge of the primary rainbow.
A Closer Look: Rainbows Are Actually Full Circles
From the ground, rainbows usually look like semicircular arches. However, that is only because the ground and horizon block the bottom half. If you look down from an airplane or a high mountaintop, you can actually see a complete circular rainbow.
Furthermore, a rainbow is not a physical object hanging at a fixed spot in the sky. It is an optical pattern formed along a 42-degree cone centered on an imaginary axis running from the sun through your eyes. As you move, your rainbow moves with you, meaning you can never actually reach its end.
Sometimes, you might spot a fainter secondary rainbow outside the main one. This occurs when light reflects twice inside each droplet, flipping the order of the colors so that red is on the inside and violet is on the outside.
๐ค Common misconceptions
A rainbow is a physical arch fixed at a specific location in the sky.
A rainbow is an optical phenomenon formed relative to the observer's position and the sun. Its true shape is a full circle, and it moves along with you as you walk.
The outer bow in a double rainbow has colors in the exact same order as the primary bow.
The secondary rainbow is formed by two internal reflections inside the droplets, which reverses the color order so that violet is on top and red is on the bottom.
๐งบ Where you meet it
Water droplets in the air refract, reflect, and disperse sunlight, splitting white light into a brilliant spectrum of colors.