April 26, 2019

Ever wonder why some animals see a purple sky?

The sky isn’t blue

Spring is finally here, bringing along with it plenty of flowers, warmer weather and blue skies. But here’s the catch: the sky isn’t really blue.

Lord Rayleigh was a British physicist in the 19th century. He discovered the element Argon, wrote books on sound that are still used today and played an important role in the start of quantum mechanics. His name is also given to Rayleigh Scattering, which is the scattering of light particles into individual colors based on size. This effect can be seen when you look up at the sky and see that beautiful Carolina Blue covering horizon to horizon.

Any fan on Pink Floyd knows that when white light enters a prism, a rainbow of color comes out the other side. Together, the colors make white light, but they have their own individual wavelengths. The prism changes the speed of each wavelength, making some colors slightly change direction, spreading them out into the rainbow they are. The same happens with rainstorms, with the raindrops acting as thousands of tiny prisms spreading the sunlight’s colors across the sky.

Earth’s atmosphere acts similarly, but these individual colors hit the atoms in the air and get scattered or reflected. The colors that have higher energies and are more excited get scattered more. Blue is higher energy, and so our sky looks blue since it’s scattered more. When the sun rises or sets, it has to travel through more air, and the reds and oranges get scattered more, giving us gorgeous scenery to look at and post to Instagram.

You might be saying right now, “But wait, blue isn’t the highest energy in the rainbow! Purple is. So why isn’t our sky purple?” And you’re right, purple is higher energy than blue, so it does get scattered more. Thus, you’ve just discovered the truth bomb of this blog.

Our sky is actually purple

Purple light has higher energy, and gets scattered more than blue. But the answer to why we see blue skies isn’t a matter of physics; it’s an answer for physiology.

Think back to high school biology. Ever done an eyeball dissection? (If not, we sometimes offer that at the Museum! Come by an open one up!) Human eyes contain shadow-sensing rods and color-sensitive cones – three types of cones to be exact. These red, green and blue cones detect a range of the natural colors in light and work together to produce all the colors you see in the world.

The predominant blue and purple scattered light triggers not only the blue cones in the eye, but also a little of the red and green cones. These all act in unison and essentially average out the scatter to, you guessed it, blue.