Dictionary of Physics : G to L


galaxy a slowly rotating system of several billion stars, gas and dust
galvanometer an instrument used to measure small electric currents
gamma camera a system of scintillators with sophisticated electronics which
images gamma rays
gamma radiation high energy electromagnetic radiation emitted by a nucleus after
emitting a radioactive particle.
gas giants planets like Jupiter, Saturn, Uranus and Neptune, which have vast
atmospheres of methane
generator an instrument that rotates a series of loops of a conductor in a
magnetic field thus producing a current
geographic north the direction pointing towards the north pole of the axis of the
Earth’s rotation.
geostationary orbit an orbit in which a satellite remains in a fixed position above the
Earth’s surface
g-force gravitational force
giant a star with a radius larger than that of a main sequence dwarf,
but less than that of a supergiant of the same temperature
GPS global positioning a satellite system which provides your longitude, latitude and
system altitude through a hand-held transponder
graded index fibres (GRIN) fibres in which the core has a refractive index that varies from the
centre outward
granulation the mottled appearance of the Sun’s photosphere caused by
convection currents
gravitational collapse the infall of a stellar atmosphere under the influence of
gravitational forces
gravitational force the attractive force between two masses
gravitational slingshot the use of the angular momentum of a large planet to accelerate
a satellite or planetary probe
gravity the attractive force between two masses
greenhouse effect the raising of planetary temperature due to the blanketing effect
of its atmosphere
ground state the lowest energy state of an atom, in which the electrons are
most tightly bound to the nucleus
gyroscope a large, rotating mass mounted so as to be able to pivot about
three perpendicular axes


half-life the time for half a sample of a radioactive nuclide to decay
harmonics the fundamental frequency and tones whose frequencies are
integral multiples of the fundamental frequency
heat thermal energy
heliocentric centred on the Sun, e.g. a heliocentric orbit
helium flash a late stage in the evolution of a star when the helium core ignites
Hertzsprung–Russell a plot of luminosity (or absolute magnitude) versus spectral type
diagram (colour or temperature)
Hubble constant a measure of the expansion rate of the universe. The slope of the
plot of galaxy recession velocity versus distance; H = 75 km s–1
per Mpc
Hyashi track the path in the Hertzsprung–Russell diagram followed by
protostars during their early gravitational collapse


impulse the product of a force and the time interval through which it acts
incoherent fibres a bundle of optical fibres in which the relative positions of the
input surfaces are jumbled with respect to the output
induced magnetism any phenomenon in which a material exhibits magnetic effects
as a result of an outside magnetic field provided by another
magnet or current-carrying coil
inductance the property of a varying current that induces a back EMF, or a
current in a conductor
induction coil a device for converting low voltage DC to high voltage DC
inelastic collision the impact of two or more bodies without loss of momentum but
in which kinetic energy is not conserved
inertia a property of matter that resists changes in motion
inertial reference frame a non-accelerating reference frame, usually based on the distant
stars, in which Newton’s first law of motion is valid
inflationary period a period soon after the Big Bang in which the universe was
expanding at a rapid rate before settling down to the present rate
of expansion
inner planets the rocky, terrestrial planets Mercury, Venus, Earth and Mars
insulator a material through which electric charge does not pass
intensity the power per square metre associated with a wave; intensity
units are W m2
interface a surface between two different substances e.g. air–glass
interference the superposing of two or more wave trains
interferometry an instrument which uses interference between the signals
received from two or more waves to reconstruct the source of the
waves; most often used in radio astronomy
intrinsic variable a variable star in which changes in brightness are caused by
internal variations, such as oscillations in radius and temperature
inverse square law the intensity associated with any spherical wave front decreases
as the inverse of the radius squared: I = Io/R2
ion a charged atom
ionisation an atomic interaction in which the atom is stripped of one of its
ionising power the energy associated with a sub-atomic particle or energetic
photon measured in terms of the number of ionisations of
hydrogen it can produce
ionising radiation any electromagnetic radiation energetic enough to ionise an
ionosphere the upper atmosphere where a large proportion of the gas exists
as ions and free electrons
irregular galaxy a galaxy with an irregular shape with no obvious spiral arms
isotope atoms whose nuclei contain the same proton number but
different numbers of neutrons


Josephson junction a junction between two superconductors separated by a narrow


Kepler’s laws 1: Planets describe an ellipse with the Sun at one focus.
2: The line from a planet to the Sun describes equal areas in equal
3: The squares of the periods are proportional to the cube of the
mean distance from the Sun.
kinematics a description of how objects move
kinetic energy energy due to motion


Larmor frequency the angular frequency of precession of an electron spin vector in
an magnetic field
laser a light amplifier that produces coherent light (Light Amplification
by Stimulated Emission of Radiation)
law of conservation: the total angular momentum of a system is the same before and
angular momentum after an experiment (collision)
law of conservation: charge the total charge of a closed system is constant throughout an
law of conservation: mass the total mass of a closed system is constant throughout an
law of conservation: the sum of the potential and kinetic energies of a system are the
mechanical energy same before and after an experiment
laws of conservation: the total momentum of a system is the same before and after an
momentum experiment (collision)
length the distance between two points along a straight line path
Lenz’s law an induced current will always flow in a direction so as to oppose
the change producing it
lepton a fundamental particle that does not interact strongly with other
particles, e.g. electron, negative muon, neutrinos
light year the distance travelled by light in 1 year
light electromagnetic radiation in the visible part of the spectrum
light, monochromatic light composed of a single wavelength
light, polarised light with vibrations in a single plane perpendicular to the ray
light, polychromatic light composed of a broad range of wavelengths (colours)
light, ultraviolet electromagnetic radiation of higher frequency (shorter
wavelength) than visible light
line spectrum a spectrum containing several discrete monochromatic
wavelengths characteristic of the element present in the source
linear accelerator an accelerator that uses a series of variations in voltage to
accelerate particles to nearly the velocity of light
linear motion motion in a straight line
liquid drop model a model of the nucleus in which it is represented as a liquid drop
governed by surface tension forces
Local Group the small cluster of galaxies of which our Milky Way is a member
longitudinal wave a compression wave in which the particles oscillate about the
direction of the wave disturbance
Lorentz contraction the change in length and time measurements made on a body
moving at close to the speed of light relative to an observer at rest
loudness a measure of the logarithm of the intensity of a sound wave
low Earth orbit, LEO an Earth orbit with radius from 100 to about 1000 km
luminosity class a stellar classification based on the width of absorption lines,
which is related to the surface pressure of the star; the
classification ranges from supergiant to sub dwarf
luminosity the power output of a star taking into account its emissions at all
Lyman series a sequence of hydrogen spectral lines seen in absorption or
emission in the ultraviolet. These result from transitions between
n = 1 and higher atomic levels.


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