Why Is the Sky Blue?

Almost every day of your life, you’ve probably looked up at the sky and seen the color blue. Perhaps there have been some days when the sky has been other colors, like snowstorms when the world above looked steely and grey, or if you’ve ever been present for a volcanic eruption or wildfire that tinted the sky an ominous orange.

Most of the time, though, a blue sky above has been one of the most reliably shared experiences among humans on Earth. Since time immemorial, we’ve looked up and seen various shades of blue when the sun is out. Also, most likely since time immemorial, kids have been asking why this is.

The answer is actually a bit complicated, so don’t worry if you are an adult who has lived your life aware of the reason behind our sky’s azure appearance! It turns out that our sky’s shade has to do with the fact that blue and violet light wavelengths scatter more frequently than any other wavelengths when they interact with molecules in our atmosphere. To unpack that sentence, we’ll need to go into some basic physics.

How Light Is Measured

Light, in short, produces energy that moves in waves. Each of these waves is a different size depending on the type of energy, and that size is referred to as a wavelength, which is measured in nanometers. The length of each wave results in different colors when the wave is perceived by the human eye. 

Violet has the shortest wavelength—380–435 nm—followed by blue, at 435–500 nm. Red has the longest wavelength, at 625–740 nm, and the rest of the rainbow is somewhere between those lengths. Together, these colors make up the visible spectrum of colors that our eyes can perceive.

How Wavelengths Create the Sky’s Color

Light moves in straight lines until it hits particles, like oxygen and nitrogen, which our Earth’s atmosphere is filled with. Contact with these particles causes the light to splinter off into random directions.

Since blue and violet have the shortest wavelengths, they scatter more frequently when they interact with atmospheric particles. This scattering effect, called Rayleigh scattering, makes these colors visible to the human eye. 

Our eyes ultimately see the sky as blue because they are more sensitive to blue light than violet light. Meanwhile, if we looked directly at the sun—which is a bad idea, by the way—we would see blinding white. This is because we’d be looking directly at light, which Isaac Newton proved contained all visible wavelengths with his famous prism experiment. 

When we look at the sky, though, we are seeing light that has been dispersed after it has already interacted with our atmosphere, which creates the beautiful blue dome above our heads.

How Scientists Discovered Why the Sky Looks Blue

This phenomenon was first identified by Irish physics professor John Tyndall, who was attempting to understand how the sun’s light caused the Earth to warm. He filled a glass tube with the particles contained in Earth’s atmosphere, and then released smoke into it. 

Then he noticed that the air looked blue when observed from the same direction from which the smoke had been released, but red when observed from the opposite side. This led Tyndall to conclude that when light interacted with dust particles in the Earth’s atmosphere, this created its color.

In 1870, British physicist John William Strutt refined Tyndall’s conclusions and confirmed that air molecules are responsible for scattering light wavelengths in our atmosphere, thus creating its color. Without this interaction, the sky would look clear, not blue, given that it is filled with colorless gases. He also developed a mathematical theory that explained why atmospheric molecules scatter shorter wavelengths more frequently than longer ones.  

Why Is the Sky Red at Sunrise and Sunset?

Every morning when the sun rises and sets, it puts on a breathtaking display of colors and shades. This phenomenon actually results from the fact that when the sun is closer to the horizon, its light has to pass through much more atmosphere to reach our eyes than it does when it’s closer overhead. 

This larger quantity of atmosphere means that blue and green wavelengths have more time to disappear as they scatter, which leads to a blending of colors and shades that results in the magnificent light shows we get to watch each morning and evening.

Why Is Space Black?

Space is black because much of it lacks an atmosphere like Earth’s, meaning that wavelengths don’t have particles to bounce off of. Of course, there are countless stars in the distance that emit light, but the fact that space doesn't look blindingly white is called Olbers' paradox.

According to modern science, space still looks black despite all the stars both because the universe has a finite age, meaning that some distant light has not reached us yet, and because the universe is expanding, which stretches light from faraway galaxies beyond visible wavelengths.

Read More: