| 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. |