Physics Glossary – U

Ultra Microscope

An instrument for viewing particles too small to be seen by ordinary microscope. A narrow beam of light focused into the liquid with a black background, the particles (less than 0.1 micron in diameter) are detected by scattering the light.


Are frequencies higher than humans can hear, in excess of 20 K Hertz. These frequencies are used in Ultra scan during pregnancy to examine the baby’s development.

Ultraviolet (UV)

Radiation is electromagnetic radiation of a wavelength shorter than that of visible light, but longer than that of soft X-rays. The name means “beyond violet” (from Latin ultra, “beyond”), violet being the color of the shortest wavelength of visible light. UV itself can be subdivided into near UV (380-200 nm wavelength) and extreme or vacuum UV (200-10 nm).

Uncertainty Principle (Heisenberg’s Uncertainty Principle)

States that one cannot simultaneously know both the position and the momentum of a given object to arbitrary precision. It furthermore precisely quantifies the imprecision. It is one of the cornerstones of quantum mechanics and was discovered by Werner Heisenberg in 1927.

Unified Field Theory

A theory that explains the four fundamental forces of nature and brings together relativity and quantum mechanics in one theory, sometimes called the Theory of Everything (TOE).

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