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Absolute humidity:

The ratio of water vapor in a sample of air to the volume of the sample.

Absolute zero:

The temperature of - 273.16 or 0 K at which molecular motion vanishes.


The ratio of the total absorbed radiation to the total incident radiation.


The rate of change of velocity with respect to time.

Acceleration due to gravity:

The acceleration imparted to bodies by the attractive force of the earth or any other heavenly body.


capable of transmitting light without decomposing it into its constituent colors.


The science of the production, transmission and effects of sound.

Acoustic shielding:

A sound barrier that prevents the transmission of acoustic energy.


Any change in which there is no gain or loss of heat.

Afocal lens:

A lens of zero convergent power, whose focal points are infinitely distant.


The fraction of the total light incident on a reflecting surface, especially a celestial body, which is reflected back in all directions.

Alpha particle:

The nucleus of a helium atom (two protons and two neutrons) emitted as radiation from a decaying heavy nucleus.

Alternating current:

The electric current that changes its direction periodically.


Solids which have neither definite form nor structure.


S.I. Unit of electric current, one ampere is the flow of one coulomb of charge per second.


The maximum absolute value attained by the disturbance of a wave or by any quantity that varies periodically.

Angle of contact:

The angle between tangents to the liquid surface and the solid surface inside the liquid, both the tangents drawn at the point of contact.

Angle of incidence:

The angle between the incident ray and the normal.

Angle of reflection:

The angle between the reflected ray and the normal.

Angle of refraction:

The angle between the refracted ray and the normal.

Angle of repose:

The angle of inclination of a plane with the horizontal such that a body placed on the plane is at the verge of sliding.


A unit of length, 1 = 10-10 m.

Angular momentum:

Also called moment of momentum, it is the cross product of position vector and momentum.

Angular velocity:

The rate of change of angular displacement with time.


A process in which a particle and antiparticle combine and release their rest energies in other particles.


The antiparticle of neutrino, it has zero mass and spin ½.

Archimedes principle:

A body immersed in a fluid experiences an apparent loss in weight which is equal to the weight of the fluid displaced by the body.

Atomic mass unit:

It is equal to one-twelfth the mass of C -12 isotope of carbon, 1 amu = 1.66x 10-27 Kg.

Atomic number:

The number of protons in an atomic nucleus.

Avogadro number:

The number of molecules in a gram molecular weight of a substance, it is equal to 6.02 x 1023.

Avogadro's law:

Under the same conditions of temperature and pressure, equal volumes of all gases contain equal number of molecules.

Balmer lines:Lines in the spectrum of hydrogen atom in visible range, produced by transition between n 2 and n = 2, n is the principal quantum no.

Bar:A unit of pressure, equal to 105 Pascals.

Baryon:subatomic particle composed of three quarks.

Beat:A phenomenon of the periodic variation in the intensity of sound due to superposition of waves differing slightly in frequency.

Bernoulli's theorem:The total energy per unit volume of a non-viscous, incompressible fluid in a streamline flow remains constant.

Beta particle:An electron emitted from a nucleus in radioactive decay.

Binding energy:The net energy required to decompose a system into its constituent particles.

Black body:An ideal body which would absorb all incident radiation and reflect none.

Black hole:The remaining core of a supernova that is so dense that even light cannot escape.

Boyle's law:For a given mass of a gas at constant temperature, the volume of the gas is inversely proportional to the pressure.

Brewster's law:States that the refractive index of a material is equal to the tangent of the polarizing angle for the material.

Brownian motion:The continuous random motion of solid microscopic particles when suspended in a fluid medium due to the consequence of ongoing bombardment by atoms and molecules.

Bulk's modulus of elasticity:The ratio of normal stress to the volumetric strain produced in a body.

Buoyant force:upward force on an object immersed in fluid.


The study of how life on Earth could have arisen from inanimate matter. It should not be confused with evolution (the study of how living things change over time), biogenesis (the process of lifeforms producing other lifeforms) or spontaneous generation (the obsolete theory of complex life originating from inanimate matter on an everyday basis).

Absolute Zero:
The lowest temperature possible, equivalent to -273.15°C (or 0° on the absolute Kelvin scale), at which point atoms cease to move altogether and molecular energy is minimal. The idea that it is impossible, through any physical process, to lower the temperature of a system to zero is known as the Third Law of Thermodynamics.

Accretion Disk:

Diffuse material orbiting around a central body such as a protostar, a young star, a neutron star or a black holeGravitycauses the material in the disc to spiral inwards towards the central body with great speed, and the gravitational forcesacting on the material cause the emission of x-rays, radio waves or other electromagnetic radiation (known as quasars).

Alpha Particle (Alpha Decay):

A particle of 2 protons and 2 neutrons (essentially a heliumnucleus) that is emitted by an unstable radioactive nucleusduring radioactive decay. It is a relatively low-penetration particle due its comparatively low energy and high mass.

Angular Momentum:
A measure of the momentum of a body in rotational motion about its centre of mass. Technically, the angular momentum of a body is equal to the mass of the body multiplied by the cross product of the position vector of the particle with its velocity vector. The angular momentum of a system is the sum of the angular momenta of its constituent particles, and this total is conserved unless acted on by an outside force.

Anthropic Principle:
The idea that the fundamental constants of physics and chemistry are just right (or “fine-tuned”) to allow the universe and life as we know it to exist, and indeed that the universe is only as it is because we are here to observe it. Thus, we find ourselves in the kind of universe, and on the kind of planet, where conditions are ripe for our form of life.


A large accumulation of antiparticles - antiprotons, antineutronsand positrons (antielectrons) - which have opposite properties to normal particles (e.g. electrical charge), and which can come together to make antiatoms. When matter and antimatter meet, they self-destruct in a burst of high-energy photons or gamma rays. The laws of physics seem to predict a pretty much 50/50 mix of matter and antimatter, despite the observable universeapparently consisting almost entirely of matter, known as the “baryon asymmetry problem”.


The basic building block of all normal matter, consisting of anucleus (which is itself composed of positively-charged protonsand zero-charged neutrons) orbited by a cloud of negatively-charged electrons, so that the positive charge is exactly balanced by the negative charge and the atom as a whole is electrically neutral. Atoms range from about 32 to about 225 picometres in size (a picometre is a trillionth of a metre). A typical human hair is about 1 million carbon atoms in width.


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Beta Particles (Beta Decay):

High-energy, high-speed electrons or positrons (antielectrons) emitted by some types of radioactive decay, when an unstable atomic nucleus with an excess of neutrons or protonsundergoes beta decay (a process mediated by the weak nuclear force). The particles emitted are a form of ionizing radiation, also known as beta rays.

Big Bang:

The huge “explosion” 13.7 billion years ago in which theuniverse (including all space, time and energy) is thought to have been created. According to this theory, the universe began in a super-dense, super-hot state and has been expanding and cooling ever since. The phrase was coined by Fred Hoyle during a 1949 radio broadcast.

Big Crunch:

One possible scenario for the ultimate fate of the universe, in which the gravity of the matter in the universe (providing that there is in fact a “critical mass”) will one day halt and reverse the universe’s expansion in a mirror image of the Big Bang, causing it to collapse into a black hole singularity. However, in the light of recent evidence for an accelerating universe, this is no longer considered the most likely outcome.

Black Body:

An idealized object that absorbs all electromagnetic radiationthat falls on it, without passing through and without reflection. The radiation emitted from a black body is mostly infrared lightat room temperature, but as the temperature increases it starts to emit visible wavelengths, from red through to blue, and then ultraviolet light at very high temperatures.

Black Hole:

The warped space-time remaining after the gravity of a massive body has caused it to shrink down to a point. It is a region of empty space with a point-like singularity at the centre and anevent horizon at the outer edge. It is so dense that no normalmatter or radiation can escape its gravitational field, so that nothing - not even light - can ever leave (hence its blackness). It is thought that most galaxies have a supermassive black hole at their heart.


Classical Physics:
A general term used to describe the physics based on principles developed before the rise of general relativity and quantum mechanics, essentially physics as it had existed up to the early years of the 20th Century. It includes the mechanics of Galileo and Newton, the electrodynamics of Maxwell, the thermodynamics of Boyle and Kelvin, and usually even the special relativity of Einstein.

The idea in quantum theory that items can be separately analyzed as having several contradictory, and apparently mutually exclusive, properties. For example, the wave-particle duality of light, wherelight can either behave as a particle or as wave, but not simultaneously as both.

Copernican Principle:
The idea that there is nothing special about our position in the universe, a generalized version of Nicolaus Copernicus’ recognition that the Earth is actually just a planet circling the Sun, and not vice versa.

Cosmic Microwave Background Radiation:

Cosmic microwave background radiation (or CMB for short) is the “afterglow” of the Big Bang, a microwave radiation which still uniformly permeates all of space at a temperature of around -270°C (about 3° above absolute zero). It is considered to be the best evidence for the standard Big Bang model of theuniverse.

Cosmic Inflation:

The idea that, in the first split-second after the Big Bang, theuniverse underwent a fantastically fast (exponential) expansion driven by the vacuum of empty space. The theory was developed by Alan Guth in the early 1980s to explain certain problems and inconsistencies with the basic Big Bang theory, such as those related to the large-scale structure of the features of the universe, the “horizon problem”, the “flatness problem” and the “magnetic monopole problem”.

Cosmic Rays:
High speed, energetic particles (about 90% of which are protons) originating from space that impinge on Earth's atmosphere. Some are generated by our own Sun, some by supernovas, some by as yet unknown events in the farthest reaches of the visible universe. The term "ray" is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles.

Cosmological Constant:
A term added by Albert Einstein as a modification to his original theory of general relativity, in order to balance the attractive force of gravity and achieve a static or stationary universe. It represents the possibility that there is a density and pressure associated with apparently empty space, and that the overall mass-energy of the universe is actually much greater than currently estimated. Once dismissed as just a mathematical “fix”, it has been revived in recent years with the discovery of the apparent acceleration of the expansion of the universe.

Cosmological Principle:

The starting point for the General Theory of Relativity and theBig Bang theory is that, that averaged over large distances, one part of the universe looks approximately like any other part, and that, viewed on sufficiently large distance scales, there are no preferred directions or preferred places in the universe. Stated in more technical terms, on large spatial scales, the universe is homogeneous and isotropic.

Critical Mass (Critical Density):

As applied to the universe as a whole, critical mass refers to the total required mass of matter in the universe which will allow the effects of gravity to overcome its continued outward expansion. If the universe contains more than the critical mass of matter, its gravity will eventually reverse the expansion, causing the universe to collapse back to what has become known as the Big Crunch. If, however, it contains insufficientmatter, it will go on expanding forever. In the same way, critical density is that overall density of thematter in the universe which will just allow continued expansion.

In other contexts, critical mass is also used to refer to the amount of fissile material needed to sustainnuclear fission.


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Dark Energy:

An invisible, hypothetical form of energy with repulsive gravitythat permeates all of space and that may explain recent observations that the universe appears to be expanding at an accelerating rate. In some models of cosmology, dark energy accounts for 74% of the total mass-energy of the universe. Its exact nature remains a mystery, although Einstein’s hypothesized “cosmological constant” is now considered a promising candidate.

Dark Matter:
Matter that gives out no light and does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter. It is estimated that there may be between 6 and 7 times as much dark matter as normal, bright matter in the universe, although its exact nature remains a mystery.


The process by which bodies and quantum systems lose some of their more unusual quantum properties (e.g. superposition, or the ability to appear in different places simultaneously) as they interact with their environments. When a particle decoheres, itsprobability wave collapses, any quantum superpositionsdisappear and it settles into its observed state under classical physics.

The mass of an object divided by its volume, a measure of how much it is compacted or crowded together (e.g. air is low in density, iron is high). Boyle’s Law dictates that a substance increases in density as its pressure is increased or as its temperature is decreased.

Independent directions in space-time. We are familiar with the three dimensions of space (length, width and height, or east-west, north-south and up-down) and one of time (past-future), but superstring theory, for example, requires the universe to have ten dimensions.


Deoxyribonucleic acid (DNA) molecules consist of two long intertwined polymers of nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds, structured as the familiar double helix. DNA is responsible for the long-term storage of genetic information, and specifies the sequence of the amino acids within proteins. It is organized into structures called chromosomes, and contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The first accurate model of the structure of DNA was formulated by James Watson and Francis Crick in 1953. The genetic information from DNA is transmitted into the nucleus of cells by molecules of RNA, which controls certain chemical processes in the cell. Both DNA and RNA are considered essential building blocks of life.


Electric Charge:
A property of microscopic particles, which may be either positive (e.g. protons) or negative (e.g.electrons). Particles with the same charge repel each other, and particles with opposite charges attract each other. The field of force that surrounds an electric charge is called an electric field, and a river of charged particles flowing through a conductor is called an electric current.

Electric Field:
The field of force that surrounds an electric charge (in the same way as a magnetic field is the field of force that surrounds a magnet). Together, the electric and magnetic fields make up the electromagnetic field which underlies light and other electromagnetic waves, and changes in either field will induce changes in the other, as shown in the equations of James Clerk Maxwell.

Electromagnetic Force (or Electromagnetism):
The force that an electromagnetic field exerts on electrically charged particles. It is one of the fourfundamental forces of physics (along with the gravitational force and the strong and weak nuclear forces), and the one responsible for most of the forces we experience in our daily lives. The electromagnetic forces acting between the electrically charged protons and electrons inside atomsand between atoms are essentially responsible for gluing together all ordinary matter.
Although hugely stronger (1042 times) than the force of gravity, it is a less dominant force on larger scales because the attractive and repulsive interactions tend to cancel each other out. Like gravity, the electromagnetic force is subject to an inverse-square law, and its strength is inversely proportional to the square of the distance between the particles. The force is mediated or operated by the exchange of photons between the particles. The ‘electrostatic force’ is one aspect of the electromagnetic force, which arises when two charged particles are static (i.e. not in motion).

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