In order to understand this question, we must learn how light and the Earth's atmosphere interact with each other.
What Is The Atmosphere?
The atmosphere is the thin layer of air surrounding the earth.
The atmosphere is mainly composed of nitrogen (78%), and oxygen (21%). The next two most common gases are argon gas and water (in the form of driplets, droplets, and ice crystals). Additional gas molecules, as well as dust, soot, ash, pollen, and salt from the oceans are also present. Climate, weather, and many other factors influence the composition of the atmosphere. Following a rainstorm or near the ocean, air may contain more water. At the bottom of the atmosphere, closest to the Earth, the atmosphere is the densest (thicker). As you ascend, the permeable layer gradually thins out.
Layers Of The Atmosphere
The layers of Earth's atmosphere is made up of several secondary layers in addition to the five major layers. Troposphere, stratosphere, mesosphere, thermosphere, and exosphere are the major layers from lowest to highest. After the exosphere is the edge of outer space.
What Is Light?
"Light is a wave of vibrating electric and magnetic fields."
In the electromagnetic spectrum, visible light is that which is observable by the human eye. Sunlight and light bulbs have the appearance of white, but they are actually a combination of many colors. By splitting light with a prism, we are able to see each color of the spectrum.
A rainbow in the sky is an example of the spectrum.
The colors constantly merge into each other. Reds and oranges make up one end of the spectrum. Gradually, these shades become yellow, green, blue, indigo, and violet. A color's wavelength, frequency, and energy vary. Among the visible colors, violet has the shortest wavelength, which means its frequency and energy are the highest among the other colors. The wavelength of red light is the longest, and it has the lowest energy and frequency.
As long as it's not disturbed, light travels in a straight line through the atmosphere. If light bumps into a dust or gas molecule, a few things could happen depending on its wave length and the size of the molecule it hits.
The wavelength of visible light is much smaller than dust particles and water droplets. These large particles reflect light, or bounce light, in various directions when they collide. Since it still contains the same colors as the incident light, the reflected light appears white.
Visible light can't penetrate through gas molecules because their wavelength is smaller. Light behaves differently if it bumps into them. Light may get absorbed when colliding with gas molecules. The molecule will eventually emit or radiate light in a different direction than what it was traveling in before. The color being radiated is the same color that was being absorbed. Each color of light is affected differently, but all of the colors can be absorbed; however, higher frequency colors, like blues, are absorbed more often than lower frequency colors, like reds. This results in a process called Rayleigh scattering, named after Lord John Rayleigh, who first described this phenomenon in the 1870's.
So now the answer that we've all been waiting for! Why is the sky blue?
The sky's blue color results from Rayleigh scattering. The longer wavelengths of light travel straight through the atmosphere as they move through.
Gas molecules absorb most of the shorter wavelength light. After the blue light is absorbed, it is then radiated in different directions, getting scattered all around the sky. The scattered blue light reaches you no matter which way you look.
This same phenomenon can also answer the question of why sunsets are the gorgeous red, purple, and pink colors!
Want to practice with light yourself? Check out these simple experiments you can do at home!
“Why Is The Sky Blue?” Science Made Simple, https://www.sciencemadesimple.com/sky_blue.html. Accessed 13 September 2021.