What causes a rainbow and why is it always an arc?

A rainbow happens when sunlight hides inside raindrops and paints a colorful arc across the sky for us to see.

Think of a single raindrop as a tiny, clear glass marble floating in the air. When bright white sunlight hits it, the light doesn't just pass through; it bends and splits apart. This is called refraction. Imagine running through a narrow hallway that suddenly opens into a big room; you have to turn slightly because your speed changes direction. The raindrop slows down the light, causing it to bend.

Inside the drop, the light bounces off the back wall like a ball hitting a bumper in a game of billiards. This is called reflection. When the light leaves the drop and goes back out into the air, it bends again. During this process, the white sunlight separates into its basic colors: red, orange, yellow, green, blue, indigo, and violet. We call this separation dispersion.

You might wonder why it looks like a curve instead of a straight line. The answer lies in your eye position. You are standing with the sun behind you and rain in front of you. Only the light that bounces off specific drops at exactly the right angle (about 42 degrees) can reach your eyes.

Imagine holding a hose and spraying water into the air on a sunny day. You will see small rainbows appear around the stream. The shape is not actually a full circle; it is just an arc because the ground cuts off the bottom part. If you were flying in an airplane above the clouds, you could see the whole circular rainbow! So, every drop contributing to that bright band of color is sitting on an invisible cone pointing from your eye toward the sky, creating that familiar curved shape.