Blue Sky
The sky appears to be a colored surface above the Earth. During the day the sky’s color varies in shades of blue. At sunrise and sunset its colors may be vivid shades of red, pink, orange, yellow, and blue.
Your eyes have special light receiving cells called cones. These cells take in light and send a coded message to your brain. It is your brain that decodes the message and identifies the color as well as its shade-pale blue, bright blue, blue-green, etc….
The sky is not a surface above the Earth. Instead, when you look upward you are looking through an “ocean” of air. The gas molecules that make up the air are very small compared to dust particles. When sunlight hits these tiny gas molecules some of the light is absorbed. After a while the molecules gives off (radiates) the absorbed light but in a different directions. This is called scattering. Since blue light is scattered in all directions, no matter what part of the sky you look at blue light is coming toward you.
The air directly in front looks clear. This is because small amounts of air look clear and colorless, but when you look through miles of it, it takes on a definite color.
Discover for Yourself
Add 1 teaspoon (5mL) of instant tea or coffee to a glass of water. Stir. Hold the glass at eye level and look through the colored water. Make note of how easy it is to see through the liquid.
Using a transparent eyedropper, fill the eyedropper with the colored liquid. Look through the liquid and make note of how easy it is to see through it.
Results
The water in the eyedropper looks more transparent than does the water in the glass. It is easier to see through the water in the eyedropper.
Why?
The water in the eyedropper is the same color as the water in the glass. The difference is that you are looking through a small amount of the water when it is in the eyedropper, The thicker the layer of water the darker will be its color. The same thing is true with air. A small amount of air looks colorless. Even a mile of air looks colorless, but ten miles looks misty blue. As the depth of the air increases the color intensifies.
Remember: The color of anything depends on the color of light that the objects scatters toward your eyes.
So why are clouds white? Because they scatter all of the sunlight that hits them.
Why are rain clouds a dark gray? Because they are generally thicker and the scattered light does not exit the bottom of the cloud. Instead, the particles making up rain clouds absorbs most of the sunlight that strikes it.
Why is the ocean blue? It is not because the blue sky is being reflected from the water’s surface. Instead, like everything that sunlight hits, the water scatters blue light from the sunlight that strikes it.