Why is the sky blue is one of the most common science questions. This is interesting because I don’t recall being asked why grass is green. Actually, the answer for both questions is fundamentally the same.

While color is always listed as a physical property, things do not really “contain a color.” Instead, the color of things depends on several factors, including the following:

1. The type of light shining on the object. For the sky, the light source is the Sun. Light from the Sun is an example of visible light also called white light.

Visible light (white light) is a combination of different colors of light, which are in order of the color with the least energy to the color with the most  energy.
This order of colors is also the order of colors in a rainbow.
Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROY G BIV)

2. What part of the light shining on an object is absorbed and what part is scattered.

3. How effective your eyes are at receiving light as well as sending messages to your brain.

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….

Now about the color of the sky. First the sky is not something at a distance from you. Instead, the Earth is surrounded by a mixture of gases called air.

When sunlight hits the molecules of gas making up air, some of the light is absorbed and some scattered in all directions. The color of the sky depends on the blend of light colors that are scattered by the gas molecules. This is generally the blue colors.

Challenge:

Why is the sky’s color pink, red, orange, and/or yellow sometimes?

(Answer: No matter the color the sky or the color of anything, color depends on the color of light scattered and received by your eyes. Of course, just as important is  how efficient the cones in your eyes are at sending messages to your brain as well as how efficient your brain is at decoding the message.)

Why are clouds white? Why are some clouds dark? For answers to these and others, see  Sky Color.