Why Is the Sky Blue?

Have you ever looked up on a clear day and wondered, Why is the sky blue? It’s one of those everyday mysteries that seems simple—but the science behind it is both fascinating and beautiful.

The Science of Scattered Light

To understand why the sky is blue, we need to talk about sunlight and the Earth's atmosphere. https://www.youtube.com/watch?v=UvmWxm3nR6E

Sunlight may appear white, but it’s made up of many colors—all the colors of the rainbow. Each color has a different wavelength. For example, red light has a longer wavelength, while blue and violet light have much shorter wavelengths.

When sunlight enters the Earth’s atmosphere, it hits tiny molecules of gas and dust in the air. This causes the light to scatter in different directions. This effect is known as Rayleigh scattering, named after the British scientist Lord Rayleigh, who first explained it in the 19th century.

Why Blue Wins Out

Here’s where it gets interesting: not all colors scatter equally. Shorter wavelengths (like blue and violet) scatter much more than longer ones (like red and yellow). Blue light is scattered almost 10 times more than red light.

So, as sunlight passes through the atmosphere, the shorter blue wavelengths scatter in all directions—filling the sky with blue light from every angle.

You might ask: If violet scatters even more than blue, why isn’t the sky purple?

Good question. The answer lies in both our eyes and the sun. First, our eyes are more sensitive to blue light than to violet light. Second, some of the violet light gets absorbed by the upper atmosphere, especially by ozone. So, blue wins the visual race.

What About Sunrises and Sunsets?

When the sun is low on the horizon—during sunrise or sunset—its light has to pass through more of the atmosphere to reach your eyes. Most of the blue light has already been scattered away by then, leaving behind the reds, oranges, and pinks we see in a beautiful sunset.

In Short…

• Sunlight contains all colors, each with a distinct wavelength.

• Blue light has a short wavelength and scatters more than other colors.

• Our atmosphere scatters blue light in all directions, making the sky look blue.

• Our eyes are better at seeing blue than violet, so that’s the color we perceive.

The next time you look up and see that endless blue above, you’ll know: you’re looking at scattered sunlight, painted across the canvas of our atmosphere by the laws of physics.