Glossary -- F

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thin material that is transparent to only a narrow range of wavelengths of light.
flat universe
a universe that stop expanding only after an infinite amount of time (a special case of an open universe).
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.
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.
any action or influence that causes an acceleration.
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 updated: 27 May 2001

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel