Oxygen is a colorless gas, which supports most life on earth. At room temperature the gas is quite inert, but most life forms are capable of using the gas for a controlled and slow oxidation process, supporting their life. In non-biological chemical reactions, the gas only reacts at elevated temperatures with combustible materials. As the gas is colorless, it cannot be displayed easily, one could equally well display a bottle filled with air.

Displaying a gas discharge also is not easily done with oxygen. The picture above shows a tube, filled with oxygen at low pressure, of approximately 20 cm length, which is connected to a 3300 V AC source, in series with a 400 kΩ resistor in order to limit the current. Still, under these conditions, hardly any emission of light can be observed, while the tube becomes very hot. Probably, infrared light is emitted, or purely high UV light, which is not visible, nor captured by the camera.

In the dark, however, one can see the emission of visible light, albeit faintly. Below, a picture is shown of the same tube, connected to the same power supply, but in an almost dark room. Exposure time was 1 second, aperture f2.8. The resulting image shows the color of the emitted light. It is a pale blue color, with a purple tinge. While making this picture, the tube becomes very hot, so most energy is converted to invisible radiation.