AstroGlossary - F
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focus
1) To bring beams of light into a convergent point, or focal point. An image created by beams of light at the focal point is said to be "in focus." Focused images have sharp edges, as opposed to being fuzzy or blurry. In smaller telescopes, astronomers place an eyepiece close to the focus in order to project the telescope focal point onto an observers eye. In larger telescopes, astronomers place a CCD camera at the focal point to capture the focused light. 2) One of two fixed points inside an ellipse. The foci of an ellipse help to define the curve of the ellipse itself. frequency
The number of oscillations, or cycles, within one second. Frequency refers to anything that exhibits the properties of a wave. For example, sound waves are produced when a piano or guitar string vibrates; when this happens at a frequency of 440 times per second, we hear the specific musical note middle C. Likewise, a "vibrating" electron emits electromagnetic waves. When the electron vibrates about 600 thousand billion times in one second, it emits green light. Ocean waves also have frequencies that you can measure by counting the number of times a floating buoy bobs up and down. With a stop-watch, count the bobs for one minute (one "bob" = one cycle), then divide by 60 to calculate the frequency of the buoy in bobs per second (cycles per second). See also: wavelength
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