Why Is the Sky Blue?

Eye and sky aligned side by side in an image for St. Charles Vision in New Orleans, LA

It’s an age-old question and one that we’ve all asked at some point: why is the sky blue?

To answer this, it’s first important to understand that the white light from the sun is a mixture of all the colors of the rainbow. Isaac Newton once famously used a prism to show the spectrum of light, separating all of the colors into distinct shades. In order for humans to detect the difference between these colors, we rely on receivers in our retinas called photoreceptors. If you were to ask your optometrist, they would explain that your photoreceptors respond most strongly to red, green, and blue wavelengths, giving you your color vision.

In the context of our question about the sky, of course, we also have to keep in mind another filter that these colors are passing through besides our eyes: the atmosphere! Light passing through a clear substance that suspends particles—like the atmosphere—scatters blue wavelengths more easily than red. You can witness this phenomenon when light passes through a glass of water as well; the red and orange light passes straight through, but the blue light is scattered in all directions.

In a similar fashion, when the sun is high overhead, the sky appears blue because the molecules in the air scatter away from the sun fairly evenly in all directions. In the evening, however, when the sun is low, its light must pass through more of the atmosphere before it reaches your eyes, giving the blue light a chance to scatter long before you see it. Because it takes a while to scatter through the atmosphere prior to reaching your eyes, what you see is a mix of evening oranges and reds. This is because the blue light has already been scattered away from the sun, hence the colors that we associate with that of a gorgeous sunset.

Next time you see the optometrist for your annual eye exam, ask to view light through a lens or prism to see how different colors are scattered. Seeing firsthand how this works is both interesting and a fun to experiment — and it gives you a small example of what you see every day on a larger scale right overhead!