Tell Me Why the Sky Is Blue
The Kid's Answer
Ruby, age 4: The sky is blue because Mother Nature made it and she didn't want it red or green or orange because I think she liked it blue. I think that's her favorite color. If it was red we could have a party, but if it were blue we could do somersaults and have a big party.
The Parent's Answer
Paula, Ruby's mom: I think I remember reading something about Mother Nature liking it blue, but I also think it has to do with the helium or hydrogen or some gases up there and the way the light filters through them.
The Scientist's Answer
Mother Nature does indeed like blue best — Ruby got that right — though Mother's preference is no whim; it's backed by some good hard science. As Isaac Newton first showed when he held a glass prism up to the light, sunlight is shot through with color. "Sunlight actually contains all the colors of the rainbow, gradations on red, yellow, green, and blue light," explains Linda Hermans, a senior engineer and astronomer at the California Institute of Technology in Pasadena. The different colors possess different wavelengths. Blue light waves are short and travel fast. When light from the sun meets our oxygen-rich atmo-sphere, oxygen molecules scatter the shorter wavelengths more easily than the longer ones. This makes blue light and oxygen a good "match," you might say. So, on sunny days when the sky is blue, it's because particles in the air scatter blue light from the sun better than they scatter the other colors.
How to Explain It to Kids
To bring the explanation down to earth for your child, offer a simple comparison: Think of how you paint the sky. You whoosh that blue paint across the paper. It's not unlike what the sun is doing, except instead of using paint and a brush, it uses light and air. Blue is the sun's "best" color; when mixed with air, it stands out the most, and the sun spreads blue light effortlessly across the giant paper of the sky. Or show them Newton's trick: On the next bright day, look at the sky through a prism. "You can spread the sunlight into all its colors," says Hermans.
And then you can do somersaults.