P
Parabola
The mathematical curve whose graph has y proportional to x2.
Parallel Beam
A parallel beam is one which does not diverge or converge - its width stays the same as the wave (which could be light or microwaves for example) travels out from its source. Parallel beams are produced using curved (parabolic) reflector dishes. Parallel light beams can also be produced using convex lenses (e.g. in binoculars or telescopes). Parallel beams can be used to send waves over much greater distances than would otherwise be possible.
Parallel Circuit
In a parallel circuit, each component is connected directly across the terminals of the power supply. Therefore, the voltage across any component is the same as the voltage across the power supply terminals.
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.
Peak
This refers to waves. The peak of a wave (also called the crest) is simply the part of the wave with the greatest amplitude.
Peak Voltage
This is the maximum voltage reached by an a.c. source in each cycle. It is very similar to the peak or crest of a wave.
Period
The time required for one cycle of a periodic motion (q.v.).
Periodic motion
Motion that repeats itself over and over.
Periodic wave
A wave in which the particles of the medium oscillate continuously about their mean positions regularly at fixed intervals of time is called a periodic wave.
Permeability
The ability to transmit fluids through openings, small passageways, or gaps.
Persistence of Vision
This is the effect which allows the human brain to be fooled into thinking that the 25 frames per second of TV is actually continuous motion. The retina retains each image for a fraction of a second and so provided that the changes between the frames are small enough, smooth motion will be seen. (The phosphor dots which make up the screen also continue to emit light for a short time after the electron beam has passed - this effect also helps to give the impression of a smooth motion).
Photographic_film
Photographic film can be used to detect visible light, x-rays, or alpha, beta and gamma radiation. These radiations will blacken film due to a chemical reaction.
Phase change
The action of a substance changing from one state of matter to another; a phase change always absorbs or releases internal potential energy that is not associated with a temperature change.
Phase
The particles in a wave, which are in the same state of vibration, i.e. the same position and the same direction of motion are said to be in the same phase.
Phases of matter
The different physical forms that matter can take as a result of different molecular arrangements, resulting in characteristics of the common phases of a solid, liquid, or gas.
Photon
A particle of light.
Photoelectric effect
The ejection, by a photon, of an electron from the surface of an object.
Physical change
A change of the state of a substance but not the identity of the substance pitch the frequency of a sound wave.
Planck's constant
Proportionality constant in the relationship between the energy of vibrating molecules and their frequency of vibration; a value of 6.63 x 10^-34 joule-sec.
Plasma
A phase of matter; a very hot gas consisting of electrons and atoms that have been stripped of their electrons because of high kinetic energies.
Plastic strain
An adjustment to stress in which materials become molded or bent out of shape under stress and do not return to their original shape after the stress is released.
Polarized
Light whose constituent transverse waves are all vibrating in the same plane; also known as plane polarized light.
Polaroid
A film that transmits only polarized light.
Positive electric charge
One of the two types of electric charge; repels other positive charges and attracts negative charges.
Positive ion
Atom or particle that has a net positive charge due to an electron or electrons being torn away.
Potential Divider (potentiometer)
A variable resistor, when used with all three of its terminals, it is called a potentiometer or potential divider. This arrangement is for controlling voltage.
Potential Energy
Potential energy means stored energy. Some examples are chemical, elastic, nuclear and gravitational - all forms of potential energy.
Power
Power is the rate of transfer of energy. The SI unit of power is the watt (W). One watt is the same as one joule per second.
The formula which relates power (P), energy (E) and time (t) is:
E = P x t
In electrical applications, there are other equations for power, which involve current, voltage and resistance. They are:
P = I V
P = I2R
P = V2 / R
Power (of a lens)
Lens power describes the curvature of a lens. High power lenses have a short focal length and are highly curved. A positive lens power indicates a convex lens and negative power indicates a concave lens. Lens power, P, is measured in Dioptres (D) and is related to the focal length, f, in metres through the formula:
P = 1 / f
Power Gain
Power gain has no units of its own. It expresses how many times more power the output signal from a process has compared to its input power. A closely related concept is that of voltage gain. Power Gain = Output Power / Input Power.
Presbyopia (presbyopic eye)
A form of farsightedness occurring after middle age, caused by a diminished elasticity of the crystalline lens.
Pressure
Defined as force per unit area; for example, pounds per square inch (lb/in^2).
Primary Colours
There are three primary colours (of light): red, green and blue. Any two of these colours can be mixed to give a secondary colour as follows:
Red + Green = Yellow
Red + Blue = Magenta (pink)
Green + Blue = Cyan (Turquoise)
If all three colours are mixed together, white light can be obtained.
These are the primary and secondary colours of light. The primary and secondary colours of pigment (paint) are not the same.
Primary coil
Part of a transformer; a coil of wire that is connected to a source of alternating current.
Primary Mirror
In a reflecting telescope, the primary mirror collects the light from the objects under study in the night sky. Reflectors can be made much bigger than the objective lenses of a refracting telescope. They are also easier to make, since their is only one optical surface to prepare. The primary mirror itself is usually made from glass, with an aluminium coating on its forward face. The mirror is made as large as possible for the same reasons as for the objective lens of a refractor.
Principle of calorimetry
If no heat is lost to the surroundings and there is no change of state also.
Principle quantum number
From quantum mechanics model of the atom, one of four descriptions of the energy state of an electron wave; this quantum number describes the main energy level of an electron in terms of its most probable distance from the nucleus.
Principle of Reversibility
This applies to light rays passing through any system of mirrors or lenses. Basically, it means if a ray of light follows a path when going, say, from left to right, then another ray of light going from right to left will follow the same path in reverse.
Probability
The likelihood that something will happen, expressed as a number between zero and one.
Probability distribution
A curve that specifies the probabilities of various random values of a variable; areas under the curve correspond to probabilities.
Projectile
An object thrown into space either horizontally or at an acute angle and under the action of gravity is called a projectile. The path followed by a projectile is called its trajectory. The horizontal distance traveled by a projectile is called its range. The time taken by a projectile from the moment it is thrown until it touches the ground is called its time of flight.
Proof
A measure of ethanol concentration of an alcoholic beverage; proof is double the concentration by volume; for example, 50 percent by volume is 100 proof.
Properties
Qualities or attributes that, taken together, are usually unique to an object; for example, color, texture, and size.
Proportionality constant
A constant applied to a proportionality statement that transforms the statement into an equation.
Proton
Protons are sub-atomic particles. They carry a positive electrical charge (qp = +1.602 x 10-19 C). They have a mass of mp = 1.673 x 10-27 kg.
Pulse
A wave of short duration confined to a small portion of the medium at any given time is called a pulse. A pulse is also called a wave pulse.