Why is the sky blue?

It’s one of the first “why” questions almost every kid asks, and the answer involves sunlight, air and the way your eyes work.

Sunlight is a mix of colors

Sunlight looks white, but it is actually a blend of every color of the rainbow. You can see this when light passes through a prism or raindrops and splits into red, orange, yellow, green, blue and violet.

Each color travels as a wave with a different wavelength. Red light has long, lazy waves. Blue and violet light have short, tightly packed waves.

Air scatters blue light the most

Earth’s atmosphere is mostly nitrogen and oxygen molecules, which are far smaller than the wavelength of visible light. When sunlight hits these molecules, some of it gets bounced off in random directions. This is called scattering.

The key detail: shorter wavelengths scatter much more strongly. Blue light is scattered roughly five times more than red light. Physicists call this Rayleigh scattering, after the British scientist Lord Rayleigh, who explained it in the 1870s.

Because blue light is scattered all over the sky, it reaches your eyes from every direction you look. The result is a blue sky.

So why isn’t the sky purple?

Violet light has an even shorter wavelength than blue, so it scatters even more. So why don’t we see a violet sky? There are three reasons:

• The sun gives off less violet light than blue light.
• Some violet light is absorbed high in the atmosphere.
• Human eyes are much more sensitive to blue than to violet.

Put together, our brains read the scattered light as sky blue.

Why sunsets turn orange and red

At sunrise and sunset, the sun sits low on the horizon. Its light has to travel through much more air to reach you than it does at noon.

Along that long path, most of the blue light gets scattered away before it arrives. What’s left are the longer wavelengths: orange, red and pink. Dust, smoke and tiny water droplets in the air can make these colors even more intense, which is why sunsets after wildfires or volcanic eruptions can look dramatic.

What about other planets?

The color of a sky depends on what’s in the atmosphere. On Mars, fine reddish dust gives the daytime sky a butterscotch tint, and Martian sunsets actually look bluish. On the Moon, there is almost no atmosphere at all, so the sky stays black even in full sunlight.

The short version

The sky is blue because air scatters short blue wavelengths of sunlight more than any other color, filling the sky with blue light. Next time someone asks, you can tell them it’s all thanks to Rayleigh scattering.