Why liquid oxygen has a blue tint
Oxygen molecules do not absorb all colors of visible light equally. They absorb some red light, so light that travels through a sufficiently thick layer emerges with relatively more blue. The liquid’s color is therefore not a blue dye or a reflection of the sky; it comes from the way oxygen itself interacts with light.
The key is how often light encounters oxygen molecules. In a thin layer, most light passes through without enough red being absorbed for the tint to stand out. In a deeper pool, light travels farther through the liquid and crosses many more molecules, allowing the selective absorption to become visible.
Why density changes what you see
Oxygen also absorbs visible light unusually weakly compared with many colored substances. Its relevant molecular transitions—the changes in a molecule’s energy state that can take in light—are difficult to trigger. In dense liquid oxygen, close interactions between molecules make that absorption more effective than it is in a sparse gas. A long enough path through oxygen gas can show a faint blue tint too, but ordinary short paths usually make it hard to notice.
That difference is a useful reminder about color: a substance need not be strongly colored on its own. Give light a long enough journey through it, and a subtle preference for absorbing one part of the spectrum can become visible.
