You've probably looked at the sky almost every day of your life and seen it blue. Perhaps there have been days when the sky was a different color, such as during snowfalls, when the world above seemed steely gray, or if you've ever witnessed a volcanic eruption or forest fire, which turned the sky an ominous orange.
However, more often than not, a blue sky above is one of the most common and frequently shared experiences among people on Earth. Since time immemorial, we've looked at the sky and seen various shades of blue when the sun shines. And, most likely, since time immemorial, children have wondered why this happens.
The answer is actually quite complex, so don't worry if you're an adult who's known the reason for our sky's azure color your whole life! It turns out the sky's hue stems from the fact that blue and violet wavelengths of light are scattered more frequently than any other wavelength when interacting with molecules in our atmosphere. To understand this, we'll need to turn to some basic physics.
How is light measured?
Excerpt from the article "Visible Solar Spectrum" | Encyclopedia Britannica/GettyImages
Light, in short, produces energy that travels in waves. Each of these waves has a different length depending on the type of energy, and this length is called the wavelength, measured in nanometers. Each wavelength results in the different colors perceived by the human eye.
Violet has the shortest wavelength—380–435 nm—followed by blue, with a wavelength of 435–500 nm. Red has the longest wavelength—625–740 nm—and the rest of the rainbow colors fall somewhere in between. Together, these colors make up the visible spectrum of colors our eyes can perceive.
How Wavelengths Create the Color of the Sky
Refraction of rainbow light by a prism | Brian A. Jackson/Shutterstock
Light travels in straight lines until it encounters particles such as oxygen and nitrogen, which fill the Earth's atmosphere. Contact with these particles causes the light to split into random directions.
Because blue and violet colors have the shortest wavelengths, they scatter more frequently when interacting with atmospheric particles. This scattering effect, called Rayleigh scattering, makes these colors visible to the human eye.
Ultimately, our eyes see the sky as blue because they are more sensitive to blue light than violet. Meanwhile, if we looked directly at the sun—which, by the way, is a bad idea—we would see a blindingly white light. This is because we would be looking directly at light, which, as Isaac Newton demonstrated in his famous prism experiment, contains all visible wavelengths.
However, when we look at the sky, we see light that has been scattered after interaction with our atmosphere, creating a beautiful blue dome above our heads.
How scientists discovered why the sky appears blue
A blue cloudy sky over an open country road | ARTpok / Shutterstock
This phenomenon was first discovered by Irish physics professor John Tyndall, who was trying to understand how sunlight warms the Earth. He filled a glass tube with particles found in the Earth's atmosphere and then blew smoke into it.
He then noticed that the air appeared blue when viewed from the same direction as the smoke emitted, but red when viewed from the opposite direction. This led Tyndall to conclude that the air's color was determined by the interaction of light with dust particles in the Earth's atmosphere.
In 1870, British physicist John William Strutt refined Tyndall's findings and confirmed that air molecules are responsible for scattering light waves in our atmosphere, thereby creating its color. Without this interaction, the sky would appear clear rather than blue, as it is filled with colorless gases. He also developed a mathematical theory explaining why atmospheric molecules scatter shorter wavelengths more often than longer ones.
Why is the sky red at sunrise and sunset?
Sunset over trees and plains in a red sky | KensCanning / Shutterstock
Every morning, as the sun rises and sets, it creates a breathtaking display of colors and shades. This phenomenon occurs because when the sun is closer to the horizon, its light must pass through a much greater layer of atmosphere to reach our eyes than when it is closer to the sky.
The larger volume of atmosphere means that blue and green waves have more time to disperse, resulting in a mixing of colours and shades, resulting in the magnificent light shows we see every morning and evening.
Why is space black?
Stars in space on a black background | Triff / Shutterstock
Space is black because most of it lacks an atmosphere like Earth's, meaning there are no particles to reflect the waves. Of course, there are countless distant stars emitting light, but the fact that space doesn't appear blindingly white is called Olbers's paradox.
According to modern science, space still appears black despite all the stars, both because the Universe is finite, meaning some distant light sources have not yet reached us, and because the Universe is expanding, stretching the light from distant galaxies beyond the visible wavelength range.
Read more:
-
Does dopamine fasting really work?
-
The surprising truth about what space smells like.
-
Black hole islands and deserted bunkers: 7 strange things people have found on Google Earth.
-
How to Photograph the Northern Lights Like a Pro
-
States hit hardest by wildfires (map).
This article was originally published on www.mentalfloss.com under the title "Why is the sky blue?".