Glossary -- F

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flat universe

a universe that stop expanding only after an infinite amount of time (a special case of an open universe).

filter

thin material that is transparent to only a narrow range of wavelengths of light.

flux

the amount of energy passing through a given area (e.g., one square centimeter) in a second. It is the apparent brightness of an object.

focus

one of two special points along the long axis of an ellipse such that the addition of the distances (satellite to focus#1) plus (satellite to focus#2) always equals the same numerical value. It is not at the center of the elliptical orbit unless the orbit is perfectly circular.

force

any action or influence that causes an acceleration.

frequency

the number of wave crests that pass a point every second. Measured in hertz (Hz). For electromagnetic radiation, the frequency is inversely proportional to the wavelength.

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last update: 06 August 1999