Air is matter that is all around us, filling our lungs, our homes, and extending for miles above the Earth’s surface. Yet, if you look around you, it’s invisible. Why can’t we see air? The answer lies in the properties of light and the composition of the air itself.
- Air is invisible mostly because its a gas. The molecules are just too small and far apart for it to reflect back much light. Only a few gases are visible, like chlorine (greenish yellow), bromine (reddish brown), and iodine (violet).
- But, air does absorb and reflect some light outside of the visible spectrum. It is not entirely invisible in infrared or ultraviolet.
- When you get enough air (like a whole planet’s worth or liquefied air) it is faintly blue.
The Invisible Matter
Air is a mixture of various gases, primarily nitrogen (78%) and oxygen (21%), with traces of water vapor, argon, carbon dioxide, and other compounds. These gases consist of molecules that are simply too small and spread out to scatter light in the same way that larger, more densely packed particles do.
Light, which is part of the electromagnetic spectrum, interacts with matter in various ways. It can be reflected, refracted, absorbed, or scattered. When light encounters an object, what we see is the light that is reflected or scattered back to our eyes.
In the case of air, the molecules are so small that they don’t scatter or absorb visible light effectively. As a result, most light passes straight through, making the air seem invisible to our eyes. However, air isn’t equally transparent across the entire electromagnetic spectrum. In particular, it absorbs some ultraviolet light, infrared, and x-rays. It’s no coincidence that human eyes don’t perceive this light. Our eyes evolved to give us the best visibility for the medium around us.
Seeing Air Indirectly
While we can’t see air directly, there are ways to observe it indirectly.
Scattering: While air doesn’t scatter light very effectively, it does scatter some light—especially light of shorter wavelengths, like blue and violet light. This scattering of sunlight by the atmosphere is why the sky appears blue during the day.
Seeing Its Effects: We can see the effects of air, such as when it moves objects around. When you see leaves rustling in a tree, that’s the air in motion.
Condensation: When air cools down, the water vapor condenses into tiny droplets that form clouds, fog, or mist. These droplets are large enough to scatter light effectively, making them visible.
Pollution and Dust: Small particles like dust, smoke, or pollutants in the air can scatter and absorb light, making the air appear hazy.
Can We Ever See Air
Air is invisible under normal circumstances. However, when it is compressed or heated, it becomes visible. For example, the air in a mirage shimmers due to light being refracted, or bent, by heated air. The distortion you see above a hot road on a summer day is another example of air becoming visible due to temperature differences. Compressed and liquefied air has enough density that you can see its color, which is blue.
The Color of the Earth’s Atmosphere
From space, the Earth’s atmosphere appears as a thin, blue halo. This is due to the scattering of sunlight, which is known as Rayleigh scattering. Blue and violet light are scattered more than other colors because they travel in shorter, smaller waves. This scattered blue light is what we see when we look at the sky.
References
- Armarego, W. L. F.; Perrin, D. D. (1996). Purification of Laboratory Chemicals (4th ed.). Butterworth-Heinemann. ISBN 978-0750628396.
- Edlén, Bengt (1966). “The refractive index of air”. Metrologia. 2 (2): 71–80. doi:10.1088/0026-1394/2/2/002
- Lutgens, Frederick K.; Tarbuck, Edward J. (1995). The Atmosphere (6th ed.). Prentice Hall. ISBN 0-13-350612-6.