A sound barrier that prevents the transmission of acoustic energy.
Any change in which there is no gain or loss of heat.
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.
The nucleus of a helium atom (two protons and two neutrons) emitted as radiation from a decaying heavy nucleus.
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.
Also called moment of momentum, it is the cross product of position vector and momentum.
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.
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.
The number of protons in an atomic nucleus.
The number of molecules in a gram molecular weight of a substance, it is equal to 6.02 x 1023.
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.
Abiogenesis: 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.
Diffuse material orbiting around a central body such as a protostar, a young star, a neutron star or a black hole. Gravitycauses 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-energyphotons 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-chargedprotonsand zero-chargedneutrons) orbited by a cloud of negatively-chargedelectrons, 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.
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.
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 holesingularity. However, in the light of recent evidence for an accelerating universe, this is no longer considered the most likely outcome.
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.
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.
General (lot. generalis - umumiy, bosh) - qurolli kuchlardagi harbiy unvon (daraja). Dastlab, 16-a.da Fransiyada joriy qilingan. Rossiyada 17-a.ning 2-yarmidan maʼlum. Oʻzbekiston qurolli kuchlarida G.
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.
Complementarity: 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.
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.
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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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.
Density: 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.
Dimensions: 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 chargedprotons 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).
A wave that travels though space at the speed of light, consisting of an electrical field that periodically grows and dies, alternating with a magnetic field that periodically dies and grows. Electromagnetic waves carry energy and momentum, which may be imparted when it interacts with matter.
In order of increasing frequency, the electromagnetic spectrum includes radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma rays.
A negatively-charged sub-atomic particle. It is an indivisible,elementary particle, and is usually to be found orbiting thenucleus of an atom. Electrons in an atom (which exist in the same quantity as the number of protons in the nucleus of the particular atom, so that the overall electric charge is zero) are constrained to occupy certain discrete orbital positions or “shells” around the nucleus. Interactions between the electrons of different atoms play an essential role in chemical bonding and phenomena such as electricity, magnetism and thermal conductivity. The discovery of electrons is credited to the British physicist J. J. Thomson in 1897.
A substance that cannot be reduced any further by chemical means. It is a pure chemical substance composed of atomswith the same atomic number (i.e. the same number of protonsin its nucleus). There are 92 naturally occurring elements on Earth, and all chemical matter consists of these elements (although a further 25 have been discovered as products of artificial nuclear reactions). Elements with atomic numbers 83 or higher are inherently unstable, and undergo radioactive decay. The list of elements is usually shown in the form of a Periodic Table, in order of their atomic number (see box at right, or click ther source link for a more detailed interactive Periodic Table).
A particle with no substructure (i.e. not made up of smaller particles) and which is therefore one of the basic building blocks of the universe from which all other particles are made.Quarks, electons, neutrinos, photons, muons and gluons (along with their respective antiparticles) are all elementary particles;protons and neutrons (which are made up of quarks) are not.
Energy: Sometimes defined as the ability to do work or to cause change, energy is notoriously difficult to define. In accordance with the Law of Conservation of Energy, energy can never be created or destroyed but it can be changed into different forms, including kinetic, potential, thermal, gravitational, sound, light, elastic and electromagnetic. The standard scientific unit of energy is the Joule.
The phenomenon in quantum theory whereby particles that interact with each other become permanently dependent on each other’s quantum states and properties, to the extent that they lose their individuality and in many ways behave as a single entity. At some level, entangled particles appear to “know” each other’s states and properties.
Entropy: A measure of the disorder of a system and of its constituent molecules. More specifically, in thermodynamics it is a measure of the unavailability of a system’s energy to do work. The Second Law of Thermodynamics embodies the idea that entropy can never decrease, but rather will tend to increase over time, approaching a maximum value as it reaches thermal equilibrium. A classic example of increasing entropy is ice melting in water until both reach a common temperature.
A one-way boundary in space-time surrounding a black hole. Any matter or light that falls through the event horizon of ablack hole can never leave, and any event inside the event horizon cannot affect an outside observer.
Exogenesis: The hypothesis that life on Earth was transferred from elsewhere in the universe. A related but more limited concept is that of panspermia, the idea that "seeds" of life exist already all over the universe, and that life on Earth may have originated through these "seeds".
Exotic Particle: A kind of theoretical particle said to exist by some theories of modern physics, whose alleged properties are extremely unusual. Examples include tachyons (particles that always travels faster than the speed of light), WIMPs (weakly interacting massive particles which do not interact withelectromagnetism or the strong nuclear force), axions (particles with no electric charge, very smallmass and very low interaction with the strong and weak forces) and neutrinos (particles that travel close to the speed of light, lack an electric charge and are able to pass through ordinary matteralmost undisturbed).
A universe which is constantly growing in size and in which the constituent parts (galaxies, clusters, etc) are flying ever further away from each other. Although contrary to the static universewhich had been assumed throughout most of history, an expanding universe was confirmed by Edwin Hubble’s 1929 observations of the redshifts of distant Cepheid variable stars, and is consistent with most solutions to Albert Einstein’s general relativity field equations. It also suggests that, in the distant past, the universe was much smaller and ultimately had its beginning in aBig Bang type event.
Fundamental (or Elementary) Forces: There are four basic forces of physics that are believed to underlie all phenomena in the universe. Listed in order of strength they are: the strong nuclear force, the electromagnetic force, the weak nuclear force and the gravitational force (or gravity). It is thought likely that, in extremely high energyconditions such as occurred near the beginning of the Big Bang, the four fundamental forces of nature are actually unified in a single theoretical framework (known as the Grand Unified Theory).
According to quantum field theory, the forces between particles are mediated by other particles, and the fundamental forces can be described by the exchange of virtual force-carrying particles: the strong nuclear force mediated by gluons; the electromagnetic force by photons; the weak nuclear force by W and Z bosons; and gravity by hypothetical gravitons.
One of the basic building block of the universe, a galaxy is a massive system of stars, stellar remnants, gas, dust, and possibly a hypothetical substance known as dark matter, bound together by gravity. Galaxies may be anywhere from 1 to 100,000 light years across and are typically separated by millions of light years of intergalactic space. They are grouped into clusters, which in turn can form larger groups called superclusters and sheets or filaments. There are many different kinds of galaxy including spiral (like our own Milky Way galaxy), elliptical, ring, dwarf, lenticular and irregular. There are estimated to be over a hundred billion galaxies in the observable universe.
A form of electromagnetic radiation produced by some kinds ofradioactive decay. Gamma rays have the highest frequency andenergy and the shortest wavelength in the electromagnetic spectrum, and penetrate matter more easily that either alpha particles or beta particles.
Gamma Ray Burst: A narrow beam of intense electromagnetic radiation released during a supernova event, as a rapidly rotating, high-massstar collapses to form a black hole. They are the brightest events known to occur in the universe, and can last from milliseconds to several minutes (typically a few seconds). The initial burst is usually followed by a longer-lived 'afterglow' emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared and radio).
Gas: A state of matter consisting of a collection of particles (molecules, atoms, ions, electrons, etc) without a definite shape or volume, and that are in more or less random motion. A gas tends to have relatively low density and viscosity compared to the solid and liquid states of matter, expands and contracts greatly with changes in temperature or pressure (“compressible”), and diffuses readily, spreading and homogeneously distributing itself throughout any container.
General Theory of Relativity:
Sometimes known as the Theory of General Relativity, this wasAlbert Einstein’s refinement (published in 1916) of his earlierSpecial Theory of Relativity and Sir Isaac Newton’s much earlierLaw of Universal Gravitation. The theory holds that acceleration and gravity are indistinguishable - the Principle of Equivalence - and describes gravity as a property of the geometry (more specifically a warpage) of space-time. Among other things, the theory predicts the existence of black holes, an expanding universe, time dilation, length contraction, gravitational light bending and the curvature of space-time. Although classical physics can be considered a good approximation for everyday purposes, the predictions of general relativity differ significantly from those of classical physics. They have become generally accepted in modern physics, however, and have been confirmed by all observations and experiments to date.
The shortest path between two points in curved space. It originally meant the shortest route between two points on the Earth's surface (namely a segment of a great circle) but, since its application in general relativity, it has come to mean the generalization of the notion of a straight line as applied to all curved spaces. In non-curved three-dimensional space, the geodesic is a straight line. In general relativity, a free falling body (on which only gravitational forces are acting) follows a geodesic in curved four-dimensionalspace-time.
Grand Unified Theory (or Unified Field Theory): Also known as Grand Unification or GUT, this refers to any of several unified field theories that predict that at extremely high energies (such as occurred just after the Big Bang), the electromagnetic, weak nuclear, and strong nuclear forces are all fused into a single unified field. Thus far, physicists have only been able to merge electromagnetism and the weak nuclear force into the “electroweak force”. Beyond Grand Unification, there is also speculation that it may be possible to merge gravity with the other three gauge symmetries into a “theory of everything”.
Gravity (or Gravitational Force):
The force of attraction that exists between any two masses, whether they be stars, microscopic particles or any other bodies with mass. It is by far the weakest of the fourfundamental forces (the others being the electromagnetic force, the strong nuclear force and the weak nuclear force), and yet, because it is a consistent force operating on all bodies withmass, it is instrumental in the formation of galaxies, stars, planets and black holes. It was approximately described by Sir Isaac Newton’s Law of Universal Gravitation in 1687, and more accurately described by Albert Einstein’s General Theory of Relativity in 1916.
Half-Life: A measure of the speed of radioactive decay of unstable, radioactive atoms. It is the time taken for half of the nuclei in a radioactive sample to disintegrate or decay. Half-lives can vary from a split-second to billions of years depending on the substance.
Random and featureless sub-atomic particles and thermal radiation predicted to be emitted by black holes due to quantum effects. Over long periods of time, as a black hole loses morematter through radiation than it gains through other means, it is therefore expected to dissipate, shrink and ultimately vanish.
The horizon of the universe is much like the horizon on Earth: it is the furthest that can be seen from a particular position. Because light has a finite speed and the universe has a finite age, we can only see objects whose light has had time to reach us since the Big Bang, so that the observable universe can be thought of as a bubble centred on the Earth.
Hubble’s Law: Formulated by Edwin Hubble in 1929, the law states that the redshift in light coming from distantgalaxies is proportional to their distance, so that every galaxy appears to be rushing away from us (or from any other point in the universe) with a speed that is directly proportionate to its distance from us. It is considered the first observational basis for an expanding universe (or the metric expansion of space), and the most often cited evidence in support of the Big Bang theory, and arguably one of the most important cosmological discoveries ever made.
The state in which the force of gravitation working to crush astar is exactly balanced by the thermal pressure of its hot gaspushing outwards. It is the reason that stars in general do not implode or explode, and it also explains why the Earth's atmosphere does not collapse to a very thin layer on the ground.
Inertia: The natural tendency (as defined in Sir Isaac Newton’s First Law of Motion of 1687) of objects to resist changes in their state of motion. Therefore, a body at rest tends to stay at rest and, once set in motion, a body tends to stay moving at a constant speed in a straight line (or along a geodesic in curved space) unless acted on by an outside force. An example of an inertial force is centrifugal force, which in reality is just due to a body trying to continue in a straight line while constrained to move along a curved path.
Inertial Frame (or Inertial System): A reference frame in which the observers are not subject to any accelerating force. An inertial frame is a frame of reference in which a body remains at rest or moves with constant linear velocity unless acted upon by outside forces (as stipulated by Sir Isaac Newton’s First Law of Motion, Force = MassCH Acceleration). Any frame of reference that moves with constant velocity relative to an inertial system is itself an inertial system.
The ability of two waves passing through each other to mingle, reinforcing each other where crests coincide and cancelling each other out where crests and troughs coincide, similar to the way ripples in water interfere with each other. This results, for example, in an interference pattern of light and dark stripes on a screen illuminated by light from two sources.
Ion: An atom or molecule that has been stripped of one or more of its orbiting electrons, thus giving it a net positive electric charge. Technically, an atom which gains an electron (thus giving it a net negativeelectric charge) is also a type of ion, known as an anion.
Isotope: A possible form of an element, distinguishable from other isotopes of the same element by its differingmass, which is caused by a different number of neutrons in the nucleus (the number of protons, which gives the atomic number of the element, must be the same). Around 75% of isotopes are stable, while some are unstable or radioactive, and will decay over time into other elements.