| Pascal: | A unit of pressure, equal to the pressure resulting from a force of 1 Newton acting uniformly over an area of 1m2. |
| Pascal's law: | The pressure exerted on a liquid is transmitted equally in all directions. |
| Paschen series: | A group of lines in the infrared region in the spectrum of hydrogen. |
| Pauli Exclusion Principle: | No two electrons in an atom can have the same four quantum numbers; thus, a maximum of two electrons can occupy a given orbital. |
| Peltier effect: | The evolution or absorption of heat at the junction of two dissimilar metals carrying current. |
| Period of oscillation: | The time required for one complete oscillation. |
| Periodic motion: | The motion which repeats itself after regular intervals of time. |
| Photoelectric effect: | The emission of electrons in some materials when light of suitable frequency falls on them. |
| Photons: | A quanta of energy in light wave; the particle associated with light. |
| Planck's constant: | The ratio of energy to frequency, equal to 6.63 x 10-34 joule-sec. |
| Plasma: | A highly ionized gas composed entirely of equal number of positive ions and electrons. |
| Plasticity: | The property of a solid whereby it undergoes a permanent change in shape or size when subjected to a stress. |
| Polarized Light: | Light whose constituent transverse waves are all vibrating in the same plane. |
| Polaroid or polarizer: | A device that produces polarized light. |
| Positron: | An elementary particle having same mass as that of an electron but equal and positive charge. |
| Potential Energy: | The energy possessed by a body by virtue of its position or configuration. |
| Power: | The rate of doing work. |
| Pressure: | The force per unit area. |
| Primary colors: | Three colors red, yellow and blue, which can be combined in various proportions to produce any other color. |
| Principle quantum number: | A quantum number that describes the main energy level of an electron in terms of its most probable distance from the nucleus. |
| Progressive wave: | A wave which transfers energy from one part of a medium to another. |
| Projectile: | An object which after being given an initial velocity is allowed to fall under the effect of gravity alone. |
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