| Balmer lines: | Lines in the spectrum of hydrogen atom in visible range, produced by transition between n 2 and n = 2, n is the principal quantum no. |
| Bar: | A unit of pressure, equal to 105 Pascals. |
| Baryon: | subatomic particle composed of three quarks. |
| Beat: | A phenomenon of the periodic variation in the intensity of sound due to superposition of waves differing slightly in frequency. |
| Bernoulli's theorem: | The total energy per unit volume of a non-viscous, incompressible fluid in a streamline flow remains constant. |
| Beta particle: | An electron emitted from a nucleus in radioactive decay. |
| Binding energy: | The net energy required to decompose a system into its constituent particles. |
| Black body: | An ideal body which would absorb all incident radiation and reflect none. |
| Black hole: | The remaining core of a supernova that is so dense that even light cannot escape. |
| Boyle's law: | For a given mass of a gas at constant temperature, the volume of the gas is inversely proportional to the pressure. |
| Brewster's law: | States that the refractive index of a material is equal to the tangent of the polarizing angle for the material. |
| Brownian motion: | The continuous random motion of solid microscopic particles when suspended in a fluid medium due to the consequence of ongoing bombardment by atoms and molecules. |
| Bulk's modulus of elasticity: | The ratio of normal stress to the volumetric strain produced in a body. |
| Buoyant force: | upward force on an object immersed in fluid. |
