Why is the Sky Blue?

By Scott Mattison (@FoolsPizza)

Blue Sky

At some point, every parent is asked why the sky is blue. A good way to explain this to a five year old is to ask them what colors make up white light, in case you forgot its the rainbow. Remind them the colors of the rainbow (red, orange, yellow, green, blue, indigo, and violet). Then explain that the air we breathe actually changes the way light moves. This is because air is made up of molecules that the light hits and changes the light’s direction. Finally, you can simply explain that the inner part of the rainbow (blue, indigo, and violet) move through the air the best to reach our eyes, giving the sky its blue color.

For the kid that keeps asking, “why” you can give them the simple answer to this question: an effect known as Rayleigh Scattering caused by small molecules in the atmosphere is what gives the sky it’s blue color. Rayleigh scattering causes shorter wavelengths to be scattered a lot more (1/wavelength^4).  Remember the colors of the rainbow from longest wavelength to shortest wavelength are red, orange, yellow, green, blue, indigo, and violet. Blue being a shorter wavelength means blue light dominates the scattered light coming from the sky.

A tricky question a kid could ask is, “why the sky isn’t violet?” This is a great question. Violet is the shortest wavelength of light we can see, so if shorter wavelengths are preferentially scattered, why isn’t the sky violet? The answer to this question is actually really cool and makes light really fun to study!

To fully explain to your (hypothetical?) child why the sky isn’t violet, first you should talk a bit more about how light scatters. The two most common ways that light scatters are Mie Scattering and Rayleigh Scattering. Mie’s solution to the scattering of light allows us to model the movement of light as it interacts with particles, like dust and water, that are similar size or larger than the wavelength. With Mie scattering, light is mostly scattered along the direction it originally hit the particle, and the effect is not wavelength dependent.

MIE
Light undergoing Mie Scattering

Rayleigh scattering, as we have already mentioned, is what gives the sky its blue color. Rayleigh scattering occurs when light interacts with particles much smaller than the wavelength size. Light that undergoes Rayleigh scattering is equally scattered in every direction. Rayleigh scattering is exponentially stronger with shorter wavelengths, meaning short blue, indigo, violet end of the spectrum is much more strongly scattered than the red, yellow, green end of the spectrum.

Rayleigh
Light Undergoing Rayleigh Scattering

Now that your (again, hypothetical ) kid knows how light scatters, you can help them begin to build an understanding of why the sky is blue. Light that is Mie scattered will mostly continue moving in the direction it was moving. So if you let your kid look at the sun (a terrible idea), they will see the effects of Mie scattering (especially if there are clouds in the way). However, if you kid looks at the rest of the sky, light that has been Mie scattered is going to keep moving away from their eyes. So overhead, Rayleigh scattering is dominant, giving the sky its blue color.

Up to this point you have only really explained to your kid the short answer for why is the sky blue. The second part of this answer relies on a the second way that light can interact with matter, absorption. I have previously talked a little about the absorption of light when I tried to figure out how to attach a laser to a shark. Here though, I am going to focus on how our bodies use the absorption of light to help us perceive the world around us.

The eyes of most people detect light via rods and cones. Rods and cones absorb light and convert the energy absorbed into a signal sent to your brain. Rods are responsible for non-color specific and night vision, whereas cones provide you with your perception of color. Most people have 3 types of cones in their eyes, one for the detection of shorter wavelengths, violet to blue, one for the perception of longer wavelengths, orange and red, and one for the perception of the middle wavelengths, green and yellow.

Rods and Cones
The light absorbance of rods and cones. Image Source: Wikipedia

Now, what colors make up white light? There are two answers to this question the first is shown on the album cover to Pink Floyd’s dark side of the moon. As we pointed out at the start, white light is made up of all of the colors of the rainbow. The second answer to this question is more familiar to anyone who works with colors on a computer. We can simulate white light with simply an even combination of red, green, and blue. In fact, we can actually simulate most colors by manipulating combinations of red, green, and blue.

So now we can get back to the question at hand, why is the sky blue? While Rayleigh scattering does preferentially scatter violet light over blue light, our eyes are far more sensitive to blue light. Additionally, Rayleigh scattering is occurring in the sky for all wavelengths of visible light, shorter wavelengths are merely scattered more. In fact what we are seeing as the color blue is really our eyes average out all of the wavelengths present in the sky and creating the perception of blue light.

Now that your inquisitive (possibly hypothetical) child has likely completely lost interest you can explain that all color is merely how our eyes perceive the world. This is why individuals who are colorblind may not realize it until much later in life. Then you can go show them other cool optical illusions that are so good are fooling us.