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How Rainbows Are Formed: A Physics Explanation
Have you ever looked up after a rainstorm and seen a beautiful arc of colors across the sky? That’s a rainbow — a natural light show created by the interaction of sunlight and water droplets. But what exactly causes this colorful phenomenon? The answer lies in physics.
What Conditions Are Needed?
To see a rainbow, three basic conditions must be met:
- Sunlight (usually low in the sky)
- Water droplets in the air (rain, mist, or spray)
- Observer with the sun behind them
When these conditions are just right, light interacts with the droplets to produce the rainbow.
The Physics Behind a Rainbow
The formation of a rainbow involves three main physical processes:
1. Refraction
When sunlight enters a water droplet, it bends because light changes speed when moving from air to water. This bending is called refraction.
2. Dispersion
As light refracts, it splits into its component colors because each color bends by a slightly different amount. This separation of white light into colors is known as dispersion.
3. Reflection
Inside the droplet, light reflects off the back surface of the droplet and travels back toward the front.
4. Refraction (Again)
As the reflected light exits the droplet, it refracts again, further separating the colors.
Why Do We See Different Colors?
The angle at which the light exits the droplet determines the color we see:
- Red light exits at an angle of about 42°
- Violet light exits at about 40°
The result is a circular arc of colors with Red on the outside and Violet on the inside:
ROYGBIV – Red, Orange, Yellow, Green, Blue, Indigo, Violet
Are Rainbows Really Arcs?
Yes and no! A rainbow is actually a full circle of light, but we usually only see the upper arc because the ground blocks the bottom half. If you’re in a plane or on a mountain, you might see a full circular rainbow.
Fun Facts
- Double rainbows occur when light reflects twice inside the droplet — the second arc has reversed colors!
- Rainbows can also appear in spray from waterfalls, fountains, or even lawn sprinklers.
Written by Vidhathri