Dictionary of Physics : G to L

G

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

H

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

I

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

electrons

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

atom

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

J

Josephson junction a junction between two superconductors separated by a narrow

gap

K

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

times.

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

L

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

experiment

law of conservation: mass the total mass of a closed system is constant throughout an

experiment

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

wavelengths

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.