On earth there are many things humans have spent decades or even centuries trying to figure out. The world is full of amazing things. Science and chemistry can often help explain many things that seem too remarkable to be true or too complex for some to figure out. Some of these things include: Why does the sky appear blue, but the air has no color? Can air ever actually be seen?
Luckily, there have been many very intelligent scientists and chemists over the centuries that have been able to solve some of the wonders of the world. In order to explain the inability to see the air one must understand what it is made up of. On planet Earth, the atmosphere is made up of many gases. Gas molecules are very far apart and smaller than the wavelength of visible light. These gases required for earth life are mainly nitrogen (78.09 percent of atmosphere) and oxygen (20.95 percent of atmosphere). There are small amounts of argon (.93 percent) and carbon dioxide (about .039 percent), as well as some other trace gases (0.003 percent). Some of the gases, such as carbon dioxide, may vary and increase as there becomes more pollution and a smaller amount of plant life to recycle that CO2 and produce oxygen. The color of air can change according to what gases are present, as well as location, weather and other particles in the air.
Everyone has seen situations where air is visible such as rainbows and the phenomenal Aurora Borealis, mainly seen in the northern hemisphere. Everyone also knows that the sky is blue and can appear multiple colors as the sun sets. Water vapor even allows air to be seen in the form of a steam cloud, and fog forms on Earth, affecting visibility when vapor condenses into liquid. So how does air appear invisible yet can be seen in some situations?
With the types of gas molecules in Earth’s atmosphere (primarily nitrogen and oxygen) and the size and distance apart, they most often do not refract nor absorb light. They also do not emit any light. If something does not interact with light in any way, it cannot be seen. Because of the gases the Earth’s atmosphere is mostly made up of, the air is colorless most of the time.
In the Earth’s atmosphere there often are other particles floating about. In the case of a blue sky, where air in a sense does appear colored, the sunlight is able to reflect off various particles and the closer to the sun the more is reflected. This is why the sky appears blue up above and gradually fades to invisible as sunlight is less able to refract. Much the same way that a flashlight's luminescence gradually fades the farther from the source the light is, the more light the more there is to reflect off of particles or objects.
Aside from the sky's appearing as different colors, there is also the strange occurrence of Aurora Borealis, where air really does seem to take on color and even appear to dance. This has been a phenomenon humans have been astounded by for as long as there have been humans. This amazing colorful wave of air occurs because of electrically charged particles from the magnetosphere of Earth. This spectacular light show occurs at an altitude of about 60 miles, rising to an altitude of about 350 miles.
Another occurrence of air being seen appears as beautiful stripes of color forming a rainbow after a rainstorm. This is possible due to water droplets in the air being able to refract multiple colors like a prism. This is why as the air dries back out the rainbow disappears.
One can gather from all of this chemistry that the only way air is ever visible is when light has something to refract off. Whether it be dust particles, water or other substances, light is able to reflect off these things. Gas molecules are simply too far apart and too small for light to be held or even bounced off of with the way beings on this planet see, so this is why air is invisible. All should be glad that air is most often invisible, as humans would always be in a colorful fog-like haze otherwise.