| Magnetic domain: | Small regions in permanent magnets within which atomic or molecular magnetic moments are aligned parallel. |
| Magnetic field: | The region around a magnet where its magnetic force is experienced by other magnetic objects. |
| Magnetic reversal: | The changing of polarity of the earth's magnetic field as the north magnetic pole and the south magnetic pole exchange positions. |
| Magnetic wave: | The spread of magnetization from a small portion of a substance where an abrupt change in the magnetic field has taken place. |
| Magnification: | The ratio of the size of the image to the size of the object. |
| Malus law: | The intensity of the light transmitted from the analyzer varies directly as the square of the cosine of the angle between the plane of transmission of analyzer and polarizer. |
| Maser: | Microwave amplification by stimulated emission of radiation. |
| Mass defect: | The difference between the sum of the masses of the individual nucleons forming a nucleus and the actual mass of that nucleus. |
| Mass number: | The sum of the number of protons and neutrons in a nucleus. |
| Mean life: | The average time during which a system, such as an atom, nucleus, exists in a specified form. |
| Mechanical energy: | The sum of energy possessed by a body due to its position, configuration and motion. |
| Mechanical wave: | The waves, which need a material medium for their propagation, e.g., Sound waves. |
| MeV: | Unit of energy, equal to 1.6 x 10-13 joules. |
| Megahertz: | Unit of frequency, equal to 106 hertz. |
| Millibar: | A measure of atmospheric pressure equivalent to 1000 dynes per cm 2. |
| Modulus of elasticity: | The ratio of stress to the strain produced in a body. |
| Modulus of rigidity: | The ratio of tangential stress to the shear strain produced in a body. |
| Mole: | The amount of a substance that contains Avogadro's number of atoms, ions, molecules, or any other chemical unit; a mole is 6.02 x 1023 atoms, ions, or other chemical units. |
| Momentum: | The product of mass and velocity of a body, it is a measure of the quantity of motion in a body. |
| Monochromatic light: | Consisting of single wavelength. |
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