SciTech

How Things Work: Why is the sky blue?

The sun emits white light that holds the wavelength of all colors. Once the light travels through the Earth’s atmosphere, it bounces through dust and water particles until it is absorbed by gaseous molecules. Because blue light has the shortest wavelength, it is the one most often absorbed and re-emitted by the gas molecules. (credit: Adelaide Cole/Art Editor) The sun emits white light that holds the wavelength of all colors. Once the light travels through the Earth’s atmosphere, it bounces through dust and water particles until it is absorbed by gaseous molecules. Because blue light has the shortest wavelength, it is the one most often absorbed and re-emitted by the gas molecules. (credit: Adelaide Cole/Art Editor)

Imagine a world in which the sky wasn't blue, but say, orange. The sky has been blue for as long as you’ve lived. But was it always this color?

To answer this question, we must understand what the Earth’s atmosphere is composed of and the properties of light.

The atmosphere of our planet is composed of a mixture of gases and a few other materials. According to space news site Universe Today, 78 percent of the atmosphere is made of nitrogen and 21 percent is made of oxygen. Other components include argon gas, water vapor, dust, and pollen. The atmosphere surrounds the entire planet, so sunlight has to pass through these gas particles on its way to Earth.

This is where the properties of light come into play. Light is energy that travels in waves moving at 300 million meters per second. The energy of radiation is dependent upon two characteristics of light: wavelength and frequency. Wavelength is the distance between the crests of a wave; frequency is the number of waves that pass by each second. Wavelength and frequency are inverses of each other.

Light from the sun appears to be white, but it is actually composed of all colors. This can be demonstrated by observing how light is split into a rainbow spectrum when it passes through a crystal prism. The colors in this rainbow all have different wavelengths, frequencies, and energies; red having the longest wavelength (lowest frequency) and violet having the shortest (highest frequency).

So what happens when sunlight passes through the atmosphere? The National Aeronautics and Space Administration’s website explains that light travels in a straight path unless something is in its way. When sunlight passes through the atmosphere, it is interrupted by the various particles. When light hits relatively large particles — such as water, dust, or pollen — it simply bounces off in different directions. However, when it hits gas molecules, which are smaller than the wavelength of light, it gets absorbed by the molecule. The molecule later radiates the same color that was absorbed.

According to NASA, although all wavelengths (colors) of light can be absorbed, shorter wavelengths (blue) are absorbed more frequently than longer wavelengths (red) because they travel in shorter waves. As a result, blue is scattered more than any other color, which is why the sky appears blue most of the time.

As the sun goes down, its light has to travel through more of the atmosphere before it reaches the planet. This means that by the time you see it, more of the blue light is already scattered, leaving the red and yellow light visible. This is why sunrises and sunsets are red.

But if shorter wavelengths are scattered more than longer wavelengths and violet light has a shorter wavelength than blue light, why doesn’t the sky appear purple? The answer, according to researchers at the University of California, Riverside, is that the physiology of the human eye makes us more sensitive to blue light than to purple light.

To return to the original question of whether or not the sky was always blue, recall that the blue color of the sky is a result of the sunlight’s interactions with the molecules in the atmosphere. This means that during a different period in our planet’s lifetime, when the atmosphere was composed of different gases, the sky may not have been the blue sky we see today.