Basis vector

　　A vector of magnitude 1 along one of the coordinate axes. Generally, we take the basis vectors to be and , the vectors of length 1 along the x- and y-axes, respectively.

　　Beats

　　When two waves of slightly different frequencies interfere with one another, they produce a “beating” interference pattern that alternates between constructive (in-phase) and destructive (out-of-phase). In the case of sound waves, this sort of interference makes a “wa-wa-wa” sound, and the frequency of the beats is equal to the difference in the frequencies of the two interfering waves.

　　Beta decay

　　A form of radioactive decay where a heavy element ejects a beta particle and a neutrino, becoming a lighter element in the process.

　　Beta particle

　　A particle, identical to an electron. Beta particles are ejected from an atom in the process of beta decay.

　　Bohr atomic model

　　A model for the atom developed in 1913 by Niels Bohr. According to this model, the electrons orbiting a nucleus can only orbit at certain particular radii. Excited electrons may jump to a more distant radii and then return to their ground state, emitting a photon in the process.

　　Boiling point

　　The temperature at which a material will change phase from liquid to gas or gas to liquid.

　　Boyle’s Law

　　For a gas held at a constant temperature, pressure and volume are inversely proportional.

　　Calorie

　　The amount of heat needed to raise the temperature of one gram of water by one degree Celsius. 1 cal = 4.19 J.

　　Celsius

　　A scale for measuring temperature, defined such that water freezes at 0ºC and boils at 100ºC. 0ºC = 273 K.

　　Center of curvature

　　With spherical mirrors, the center of the sphere of which the mirror is a part. All of the normals pass through it.

　　Center of mass

　　Given the trajectory of an object or system, the center of mass is the point that has the same acceleration as the object or system as a whole would have if its mass were concentrated at that point. In terms of force, the center of mass is the point at which a given net force acting on a system will produce the same acceleration as if the system’s mass were concentrated at that point.