Oxygen exists in our atmosphere as a transparent, colorless, and odorless gas. Specifically, the \(\text{O}_2\) molecule makes up approximately 21% of the air we breathe, yet it remains invisible under normal conditions. This invisibility is not a universal constant of the element, but rather a property of its common gaseous state. Oxygen’s visibility is subject to extreme changes in temperature, pressure, and energy, which can transform it into a pale blue liquid or even cause it to glow with vibrant colors in the upper atmosphere.
Why Gaseous Oxygen is Invisible
The transparency of oxygen gas is rooted in how its molecules interact with light from the visible spectrum. For a substance to be seen, it must either absorb, reflect, or scatter light that falls within the wavelengths detectable by the human eye (about 380 to 750 nanometers). Gaseous oxygen molecules are largely transparent because they do not absorb light in this visible range.
Oxygen molecules do absorb light, but primarily in the ultraviolet and infrared regions, which are outside of our visual perception. Visible light simply passes through the oxygen and nitrogen molecules that make up the air without being significantly absorbed. These molecules are also far too small and dispersed in the gaseous state to effectively scatter light back to our eyes.
A small amount of light scattering, known as Rayleigh scattering, does occur in the atmosphere. This scattering is what makes the sky appear blue, but it is a collective effect involving a tremendous concentration of all atmospheric molecules. The small size of the gas molecules ensures that light transmission is the dominant phenomenon, resulting in the gas itself being clear and unseen.
The Visible State of Liquid Oxygen
When oxygen is cooled to cryogenic temperatures, specifically below its boiling point of approximately \(-183^\circ\text{C}\) (\(-297^\circ\text{F}\)), it condenses into a liquid that is visibly blue. This transformation from an invisible gas to a pale, clear cyan liquid is a direct consequence of the molecules being forced into close proximity. The liquid state increases the density of the substance roughly a thousand times compared to its gaseous form.
This intense concentration allows for a unique interaction between the oxygen molecules, which contain two unpaired electrons, making them strongly paramagnetic. The close packing of molecules facilitates the simultaneous absorption of light by pairs of \(\text{O}_2\) molecules, a process known as collision-induced absorption. Liquid oxygen absorbs light in the red and yellow parts of the spectrum, allowing the blue wavelengths to be transmitted and scattered. This selective absorption causes the liquid to exhibit its characteristic pale blue color.
When Oxygen Emits Light
Oxygen can become visible by emitting light, a process that occurs when its atoms or molecules are energized and then release that energy as photons. This phenomenon is most spectacularly observed in the upper atmosphere as part of the aurora borealis, or northern lights. The aurora is created when high-speed charged particles from the sun, mainly electrons, collide with atmospheric gases.
When these electrons strike oxygen atoms, they transfer energy and bump the atom’s electrons into a higher-energy, or excited, state. The oxygen atom then releases this excess energy by emitting light as its electrons drop back to a lower-energy state. The color of the emitted light depends on the specific energy state change and the altitude at which the collision occurs.
Oxygen atoms at higher altitudes, above 300 kilometers, can emit a deep red light, corresponding to a wavelength of 630 nanometers. The most common and recognizable auroral color is a vibrant greenish-yellow glow, which is emitted by oxygen atoms at lower altitudes, typically between 100 and 400 kilometers. This green light is emitted at a wavelength of \(557.7\) nanometers and is the signature of excited oxygen in the aurora.