(c) Anthony K. Grafton 2003

Blue Sky, Red Sunset

Did you ever wonder why the sky is blue? Have you ever seen a beautiful red sunset? Both of these colorful parts of nature result from relatively simple ideas of physics and sunlight.

Sunlight is made up of many colors, and these colors can all be seen in a rainbow, but they always appear in the same order: red, orange, yellow, green, blue, indigo, and violet. (Remembering the name Roy G. Biv, will help you remember the order of the colors by the first letters of their names.) All light is made of waves, and each color of light has a different wavelength. Red light has the longest wavelength, while blue and violet light have the shortest.

Sunlight is scattered by the air in our atmosphere in much the same way that car headlights are scattered on a dusty road. Light with a short wavelength is scattered more than light with a long wavelength. So, when you look up and away from the sun during a clear day, you see mostly blue sky, since primarily blue light is scattered back toward you.

During sunrise and sunset, you see light coming through a lot of air (since its coming in sideways from the horizon) and almost straight from the sun (but don’t look directly at the sun, as it can injure your eyes!). Most of the blue light is scattered away, leaving the red and orange light for you to see as a beautiful spectacle of nature to greet the mornings and welcome the evenings.