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fall (of meteorites)  Meteorites seen in the sky and recovered on the ground. 
fault  in geology, a crack or break in the crust of a planet along which slippage or movement can take place, accompanied by seismic activity. 
field A mathematical description of the effect of forces, such as gravity, that act on distant objects. For example, a given mass produces a gravitational field in the space surrounding it, which produces a gravitational force on objects within that space.
find (of meteorites) A meteorite that has been recovered but was not seen to fall. 
fireball  A spectacular meteor, seen for more than an instant in the sky. 
fission The breakup of a heavy atomic nucleus into two or more lighter ones. 
flare  A sudden and temporary outburst of light from an extended region of the Sun's surface. 
fluorescence  The absorption of light of one wavelength and re-emission of it at another wavelength; especially the conversion of ultraviolet into visible light. 
flux The rate at which energy or matter crosses a unit area of a surface.
focal length  The distance from a lens or mirror to the point where light converged by it comes to a focus. 
focus (of ellipse)  One of two fixed points inside an ellipse from which the sum jof the distances to any point on the ellipse is a constant. 
focus (of telescope)  Point where the rays of light converged by a mirror or lens meet.
forbidden lines  Spectral lines that are not usually observed under laboratory conditions because they result from atomic transitions that are highly improbable.
Force That which can change the momentum of a body; numerically, the rate at which the body's momentum changes. 
Fraunhofer line  An absorption line in the spectrum of the Sun or of a star.
Fraunhofer spectrum  The array of absorption lines in the spectrum of the Sun or a star.
frequency  Number of vibrations per unit time; number of waves that cross a given point per unit time (in radiation). 
fusion The building up of heavier atomic nuclei from lighter ones. 
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