P

p type - a type of semiconductor where the current is carried mainly by positive holes
Pallas - the second asteroid to be discovered (1802)
parabolic - a certain shaped curve, parabolic path, parabolic mirror, parabolic orbit
parallax - the apparent movement of one object relative to another when the observer moves
parallel - side by side, as for resistors or capacitors in a parallel circuit
parity - symmetry of reflection in space
parsec - the distance of a star with a parallax of 1 second of arc (3.26 light years)
Pascal (Pa) - unit of pressure. 1 Pa = 1 N/m²
Pauli exclusion principle - two fermions in an atom cannot have the same quantum state
peak value - maximum value, biggest value
pendulum - a device that swings backwards and forwards at a regular rate, bob on a string
penumbra - partial shadow, as in an eclipse
perigee - point in orbit when a satellite is closest to the Earth
perihelion - point in orbit when a planet is closest to the Sun
period - the time taken for one oscillation or one orbit
periscope - instrument using two mirrors (or two prisms) for seeing over obstacles
permeability - property of a material to 'absorb' magnetic fields
permittivity - property of a material to 'absorb' electric fields
perpendicular - at right angles to
Perseids - major meteor shower peaking on 12^th August
persistence of vision - image lasting for a tenth of a second after viewing the object
persistent - long lasting
perturbation - a slight disturbance of the motion of one body due the presence of another
Phobos - satellite of Mars
phon - unit of sound level where all sources are compared with one of 1 kHz
phonograph - early form of 'talking machine' invented by Edison
photoelastic - effect of stress on the light passing through a transparent material
photoelectric - emission of electrons from a surface when light falls on it
photoelectron - an electron produced by the photoelectric effect
photometer - instrument for measuring the brightness of a light source
photometry - measurement of light in terms of its luminous or visible effect
photomultiplier - a tube where the initial photo-electron gives a cascade of electrons
photosphere - the bright, luminous surface of the Sun
phonon - particle with no rest mass that transmit thermal energy along a metal rod
photon - particle of light energy (quantum of light)
phosphor - material that glows when radiation or charged particles fall in it, as in TV screen
pico - one million millionth (10^-12) as in pico amp
piezo-electric - the effect where a voltage is produced by squashing a crystal
pinhole camera - a box with a pinhole in one side used for producing images or taking photographs
pitch - level of the note we hear. Depends on the frequency. Low pitch - low frequency
Planck's constant - ratio of the energy of a quantum to the frequency of that quantum
plane - flat, as in 'plane waves' - waves with a flat front
planet - body moving in an elliptical orbit round the Sun, shining by reflected light from it
planisphere - circular star map with a rotating mask to show the sky at different times of the year
plasma - mixture ('soup') of separated electrons and positive ions. e.g. in fluorescent lights
plastic - material that does not return to the original size when the stress is removed
Pleiades - very distinct open cluster in the constellation Taurus. (known as the Seven Sisters)
plössl - type of telescope eyepiece giving wide and flat field of view
Pluto - the furthest known planet from the Sun
plutonium - a poisonous fissile material used in the making of nuclear weapons
pneumatics - branch of Physics dealing with the dynamic properties of gases
poisons ratio - ratio of the transverse contraction strain to the longitudinal extension strain
polar axis - of an equatorial mounting, axis aligned with the axis of the Earth
polarimeter - an instrument to measure the rotation of the plane of polarisation of light
polarised - wave motion vibrating in one direction only (plane polarised - in one plane)
pole - point of most concentrated magnetic field of a magnet
pole star - Polaris, a star roughly in line with the N end of the Earth's axis
population inversion - energy inversion of electrons in an atom – more in higher energy states
positive feedback - feedback that tends to increase the output of a system
positron - a particle with the same mass as an electron but with a unit positive charge
potential - possibility of something happening, as in potential energy.
potential difference - voltage (unit volt)
potential divider - a device for controlling the output voltage
power - the rate at which work can be done (referred to a 'machine')
precession - rotation of the position of the orbit of a planet (e.g. Mercury) around the Sun
precession - movement of the axis of a gyroscope due to an applied torque
predict - forecast, suggest a future happening
presbyopia - reduction in the range of accommodation of the eye with age
pressure - force/area
primary - basic, foundation, first
primary colours - red, green and blue (for light)
prism - a block of transparent material with two faces inclined at angles other than 90^o
probability - chance
projectile - an unpowered object moving through a gravitational field
propagate - give out, carry
propel - push, as in 'the force needed to propel a rocket'
proper motion - the angular motion of a star across the star sphere (max is 10.3" per year)
proportional - quantities where as one changes the other changes in a constant way
proton - particle with a single positive charge. Nucleus of a hydrogen atom (simplest form).
proton number - number of protons in the nucleus - determines which element it is
proxima centauri - the nearest star, 4.2 light years from the Earth
pulsar - a rotating neutron star giving out regular pulses of radiation
pupil - the black hole in the centre of the eye through which light enters the eye
P-wave - a longitudinal mechanical vibration in the Earth's crust - primary, push
pyrometer - instrument for measuring high temperature, as in 'optical pyrometer'