Why is the Sky Blue?

September 20th 2016

Why is the Sky Blue?

Science is all around us

The constant stream of new research studies that come out of universities can sometimes make us think that science only occurs within the confines of high-tech laboratories. It is important to remember that science is constantly working around us and if we pay attention we may learn something interesting.

Take for instance, something that we often take for granted, the color of the sky. Why exactly does it appear blue on a clear day? It turns out that there are two parts to this answer:

  1. the sky (what we are studying)
  2. ourselves, the observer (what is doing the measuring).

Let’s now tackle the sky first, what exactly is it about the sky in particular that leads us to observe it as blue?

See the rainbow

Sunlight that hits the earth is white but we know from rainbows that certain objects, such as water droplets, will split the light up into their individual colors. This tells us that light coming from the sun is composed of all the visible colors.

Our atmosphere is mostly composed of nitrogen and oxygen gas molecules. Both nitrogen and oxygen are composed of two atoms of the same type that are bound together. We can think of a chemical bond as a tiny spring that vibrates. Sometimes, an incoming light ray, which is also known as a photon, can directly excite the molecule which will take it from a low energy vibrational state to a high energy, or excited, state. 

Rayleigh Scattering

Rayleigh Scattering- Light has a small probability of exciting nitrogen and oxygen molecules from a low energy to an excited state for a short period of time. The molecule will eventually relax and release a photon to ensure that energy is not created or destroyed.

A molecular vibration has similarities to a metal spring, which can also vibrate at a low and high energy state. For example, a metal spring that you pick up will vibrate at a low frequency, but if you flick it with your finger, it will vibrate more intensely before it gradually relaxes back to its initial state. A molecule that is excited will relax back to its low energy state and in doing so, it will release a new photon of the same color in a random direction.

The process whereby a photon’s direction can be randomized by a gas molecule is a process that we call scattering. This particular type is called Raleigh Scattering but other kinds exist as well.

You may have observed scattering at a laser light show. Light that leaves a laser will travel in one direction but water droplets from fog will scatter the light in different directions. We see this randomization as a glowing haze along the path of the beam. This scattering occurs with the water droplets rather than individual molecules, but the effect that we see is similar.

The important part about Rayleigh Scattering is that the chance that a light wave will interact with a molecule is based on the color of the wave. Red light has a longer wavelength than purple light and as both colors pass through the same volume of atmosphere on their way to earth, purple light will interact with more molecules than red light will which increases the chance that the purple light will scatter. We can see a simple demonstration of this below. I arranged nitrogen molecules in a random arrangement that is identical for the red and violet light, but due to the smaller wavelength of the purple light, it will have more interactions (signified by yellow stars) in the same area when compared to red light. 

Gas in atmosphere scatters more off purple light than red light.

Light with a shorter wavelength interacts with more molecules as it goes through the same volume and be more likely to be scattered. In this example, purple light interacts with 16 molecules while red light only interacts with 10 even though the arrangement of the molecules is identical.

From what we just discussed, the sky should really appear purple since higher frequency light will scatter more than lower frequency light. Since purple light is even higher frequency than blue, we should see the sky as purple right?

Vision is Subjective

The reason that we see the sky as blue instead of purple brings us to the next factor that influences how we see the sky, our eyes. Light comes into our eyes and is focused by a lens onto the retina which is a light sensitive part of the eye. Cone cells detect color and there are three types that are sensitive to blue, green, and red light. As light hits those cells, our brain will figure out what color it is based on how much the light triggers each of those three types of cone cells.

Our eyes are more sensitive to blue light than purple light.

Red light, for example will primarily excite the red cells, blue will excite the blue cells while orange will excite both red and green cells. We can see that the sensitivity for each color across the spectrum is not the same. The result of this is that we are much less sensitive to purple light than blue light so even though we would see the sky as purple if our cones were equally sensitive for all colors of light, we instead see it as blue because we have a higher sensitivity to blue than purple.

Our View of the World is Shaped by Our Senses

How we view and interpret our world is affected by the tools that we use to measure our surroundings. Even though our observations are always true, our interpretation of those observations is always subject to revision based on future studies. This lack of absolute certainty of interpretations in science forces us to always remain open to changing our interpretation if we are presented with new evidence that contradicts old interpretations. On the other hand, if an interpratation is consistent with many studies, we can be reasonably confident that it is in relative agreement with reality.

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About the Author

Stephen Jensen

Founder of Pocket Scholar, Ph.D. Surface Scientist and Clean Energy Aficionado.