Commonly, the word “vacuum” is applied to an enclosed volume containing gas at a lower pressure than the surrounding atmospheric pressure. So many applications, processes, and products involve vacuum—that attempting to classify them; is futile. However, using very broad definitions, vacuum applications fit into six categories...
1. Large Hadron Colliders
Moving electrons (or ions) from here to there as in x-ray tubes, beam lines, mass spectrometers, etc., demands high vacuum. Why? Because electrons/ions will be deflected by, attach to, or ionize any residual gas molecules they encounter. Vacuum creates conditions in which charges or uncharged particles can be moved around without collision.
Evaporating aluminum as a thin coating on glass or plastic makes a wonderful headlamp reflector, DVD, or rear-view mirror. But try evaporating aluminum in air and the result is aluminum oxide, a white substance not noted for its reflective properties. Vacuum prevents chemical reaction with air.
All good cameras lenses are coated with an anti-reflective layer so the maximum amount of light arrives at the film or digital processor. By contrast, architectural glass is coated with partially reflective layers for visible or infra-red wavelengths. Any oil or water vapor absorbed on the glass surface prior to coating ruins the process. Vacuum helps remove absorbed contaminants from surfaces.
4. Everday Products
Vacuum forming is a common process for making plastic Halloween masks, compartmented lunch trays, and disposable razors. The plastic sheet is heated to a deforming temperature and the air is removed between it and a metal mold to create a differential pressure.
5. Neon Signs
Vacuum removes air in preparation for backfilling with an appropriate gas, vapor, or liquid. Neon signs contain neon (and other gases for different colors); electrical switchgear is backfilled with SF6 to prevent arcs; and all fluorescent lights are backfilled with mercury vapor.
6. Clean Surfaces
Tribology experiments (the science of wear and friction of clean surfaces) often starts with breaking a crystal under vacuum to get a clean surface that has no absorbed contaminants. If the chamber's pressure is one millionth of an atmosphere, the initially clean surface is coated with a mono-layer of residual gas within ~1 second. If the chamber is at one billionth of an atmosphere, the time increases to ~1000 seconds. Vacuum reduces the flux of the residual gas on a surface.