Why Is the Sky Blue?

Look up on a clear day. What color do you see? Blue. But why?

The answer comes from the sun and the air.

Sunlight looks white to our eyes. But white light is actually all the colors of the rainbow mixed together. Red, orange, yellow, green, blue, indigo, violet. Each color travels in waves.

Red and yellow waves are long and lazy. They pass right through the air easily.

Blue waves are short and choppy.

Earth has an atmosphere. This is a layer of gases and tiny particles wrapped around the planet like a blanket.

When sunlight reaches Earth, it hits the gases in the atmosphere. Red and yellow light go straight through. But the short blue waves crash into the gas particles. They bounce around in every direction.

Scientists call this scattering. Because blue light scatters everywhere, you see it when you look up.

What about at sunset?

When the sun goes down, the light must travel through more air to reach your eyes. By the time it gets to you, all the blue light has scattered away. Only the long red and orange waves are left. That is why sunsets are red and orange.

Next time you look at a blue sky, you know the secret. It is just light bouncing off the air.

What color is the sky on Mars?