f-number – the ratio of the focal length of the lens to the diameter of the lens (see equation 23.5)

Fahrenheit temperature scale – a scale to measure temperature that defines the freezing point of water as 32° and the steam point as 212°

Far point – the greatest distance from the unaided eye that produces a distinct (focused) image on the retina

Faraday’s law – states that the emf induced in a loop of wire is proportional to the rate of change of magnetic flux through the loop

Farad – a measurement of capacitance equal to 1 coulomb per volt

Fermi energy – a cutoff energy; the last filled energy state (see figure 31.13)

Ferromagnetism – a magnetic effect resulting from a cooperative interaction of individual ionic moments where the ionic moments are aligned in the same direction

Field lines – a diagram of continuous lines drawn to illustrate the direction of a force on a test mass, or charge (also called lines of force)

First law of thermodynamics – - the change in internal energy of a system is the difference between the heat taken in by the system and the work done by the system (see equation 13.1)

Fission – the nuclear reaction involving the splitting apart of atoms

Fluid – any substance that cannot maintain its own shape

Forbidden zones – regions of energy in which no electron states are allowed

Focal length – the distance from the center of a lens to the focal point (the point where light striking the lens parallel to the optical axis is focused)

Force – a “push” or a “pull”

Frequency – the number of complete revolutions or cycles an object makes per unit of time

Free-body diagram – a vector diagram showing all forces that act on each object

Freely falling bodies – objects that are moving under the influence of gravity only

Fresnel lens – a very thin lens designed to be equivalent to a much thicker lens made the conventional way (see figure 23.14)

Friction – a retarding force that resists motion between systems that are in contact

Fundamental frequency – the lowest resonant frequency of vibration

Fundamental units – ( see base units ) units of measurement that establish a standard

Fusion – the nuclear reaction that results from the joining of two nuclei