Why is the sky blue? -Lanie, 10, St. Louis, MO

Dear Lanie,

Just the other day I looked up and wondered the very same thing. The sky is certainly blue, I thought. But on second thought, it isn’t always blue. Sunsets burst in pink and orange. The night sky is black.

That’s when it hit me. If our sky gets dark when the Sun is out of sight, maybe the answer to your question had something to do with light.

I decided to visit my friend Cigdem Capan, a physicist at Washington State University.

“When we look at the sky during daytime, the sky does not emit the light,” she said. “It receives it from the Sun and spreads it around. Only some of the rays will reach the surface of Earth, or our eyes.”

I wanted to know more about how it all worked. Capan explained that sunlight is actually white. It’s a mixture of all the colors of the rainbow.

Screen Shot 2016-05-06 at 4.08.12 PMIf you’ve ever seen light zipping through a prism, you know it can break into different colors—red, orange, yellow, green, blue, and violet.

But as you’ve observed, the sky is blue, not rainbow. Something must bring out blue, I thought. I wondered what that something might be.

It was actually a 19th century physicist, Lord Rayleigh, who suggested that the air itself is what might help make this happen. He thought the particles in the air made the sky blue.

Light from the Sun travels to our Earth in about 8 minutes. It moves in a straight line and really fast. That is, unless something gets in the way—something like the air in our atmosphere.

The molecules that make up air, like nitrogen or oxygen, get really energized when they meet up with the incoming light.

The molecules scatter the light, and in all kinds of directions. It’s a bit like when a bunch of Ping-Pong balls get hit by a bunch of rackets at the same time, Capan said. The molecules such as oxygen and nitrogen are like the rackets and the light would be like the Ping-Pong balls.

Rayleigh discovered that these little oxygen and nitrogen molecules could scatter blue rays much more than red rays. There are significantly more blue rays than red rays reaching us from the air, Capan said. It is the reason we see the sky as blue when the Sun is high in the sky.

Light can scatter to bring color into our world. But our brains and eyes help us translate these different rays of light into color, too. In fact, our eyes have millions of itty-bitty parts called cones and rods that help us see color. We cats don’t just see black and white. We can see colors, too. My eyes not only help me see during the day, but also in the dark of night.

As you can see—literally—when light from the Sun scatters, it makes our sky appear blue. But your question got me wondering about something else: If the sky is blue, why is a sunset orange and pink? Write me sometime and let me know what you think.


Dr. Universe