**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. |