Ask A Scientist

Why is the sky blue?

Asked by: Amanda Fedor
School: Glenwood Elementary School, Vestal
Grade: 1
Teacher: Miss Brigham
Hobbies/Interests: A famous Native American elder, Chief Joseph once said, “Good words only last for so long. I’m tired of all the good words.” Chief Joseph was so right. I know many of my students feel exactly the same way! Rather than simply talking about the concepts, let me walk you through a very simple experiment you can do on your own at home. All you need is a flashlight, a glass beaker or an aquarium, and powdered milk. And you thought science was complicated? So here is what I ask you to do. Fill the beaker or aquarium with ordinary tap water. Place the flashlight so that it shines through the water and comes out the other side. Slowly drop in the condensed milk a pinch at a time. Keep adding the condensed milk until the beam of light becomes visible throughout. Now here’s the trick. If you look at the light beam from the very end, it will appear yellow-orange. Yellow-orange? That sounds like the colors of a beautiful sunset. If you look along the side, the light will appear bluish-white. Bluish-white? That sounds like our Earth’s daytime sky to me! So what exactly is going on? Our sun produces light, which is made up of light of all the different colors of the rainbow. There is red, orange, yellow, green, blue, indigo, as well as violet. Artists speak of colors while scientists speak of wavelengths and frequencies. They may seem totally different but in fact describe the same things. Each color corresponds to a wavelength or a frequency. As we move from red to orange and eventually to blue and indigo, the wavelengths of the light decrease and the frequencies increase. When the white light from the sun shines through the earth's atmosphere, it collides with our atmosphere’s molecules. These molecules scatter the light. The shorter the wavelength or higher the frequency of the light, the more it is scattered by the atmosphere. Let’s see if we can use our practical experience to make some sense of this. Suppose you had a pogo stick and you wanted to get across a room from one doorway to the next. You could do it one of two ways. You could take short jumps and bouncing very high, perhaps even hitting your head on the ceiling. Or you could take just a few long jumps and hit your head only a few times. The short jumps with lots of collisions can be thought of as representing the blue light while the longer steps with fewer impacts can be imagined as the yellow-orange light. Certainly after a while, you would get tired of all the collisions no matter the size of the step but the shorter steps or blue light would cause you to tire more quickly than the longer steps or yellow-orange light. Light acts the same way. Rather than our head colliding with the ceiling, in science we refer to this phenomenon as light scattering. Let’s get back to our simple experiment. When we looked from the side, the blue light was scattered first and so to us the light beam took on a bluish-white glow. As the light continued to travel through the beaker, all the blue light was scattered away while the yellow-orange remained as we reached the opposite edge of the beaker. When we look up at the daytime sky, we are essentially only seeing the blue light being scattered. If we wait until sunset and look off to the setting sun, we see the only light that’s left—the yellowish-orange color. All this changes of course if our atmosphere becomes ever more polluted. What once was a beautiful daylight sky turns more and more yellowish orange overhead as the number of contaminates continues to increase.
Career Interest: Dog catcher

Answer from George Catalano

Professor of Mechanical Engineering, Director of t

Research area:
Turbulence, Fluid Mechanics, Aerodynamics, Environmental Ethics, and Modeling Ecosystems

PhD School:
University of Virginia, Aerospace Engineering, 1977

Interests/hobbies:
All things Italian, Creative Arts, Model trains & cars

Family:
Wife, Karen, is a registered yoga teacher at Yoga for Everybody at the Orthopedic Associates; lives with 3 Alaskan Malamutes, two more in our hearts

I like to play with animals

Last Updated: 9/18/13