Why the Sky Is Blue: Why Is the Sky Blue and Not Any Other Color?

This updated guide examines Why the Sky Is Blue: Why Is the Sky Blue and Not Any Other Color? and organizes the essential facts, background, and practical takeaways in clear American English.

Sunlight has 7 colors (red, orange, yellow, green, blue, indigo, purple), each color has a different wavelength, frequency and energy. In particular, purple light has the shortest wavelength but has the highest frequency and energy. Red light has the longest wavelength but the lowest frequency and energy.

Light travels in space in a straight line, when entering the atmosphere is blocked by small particles of dust or gas molecules. From here, depending on the wavelength of the light and the size of the objects it comes in, it has different colors.

In the air, dust particles and water are larger than the wavelength of visible light, so when light shines on them, they are absorbed or reflected by obstacles in different directions. The different colors in the light will all be reflected in the same direction so the light reflected from the resist particles retains the original colors.

The gas molecules in the atmosphere are smaller than the wavelength of visible light. So when white light hits a gas molecule, part of it can be absorbed and radiated by it in different directions than originally. Light with shorter wavelengths is more absorbed (like blue or purple), and long wavelengths (like red) that are not absorbed continue to pass through the Earth's atmosphere. Meanwhile, blue light will be scattered throughout the sky.

So why is the sky not purple the shortest wavelength but the blue one?

When the sky is a mixture of blue and purple, the cone cells in the human eye will see this mixture as blue and white, then the signal to the nervous system is just blue. Therefore, when people look at the sky, people will see it is green.

However, not all animals see the sky as blue as humans. For example, birds will see the sky as purple.