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A criterion for the how finely a set of optics may be able todistinguish. It begins with the assumption that central ring ofone image should fall on the first dark ring of the other.relativity principle; principle of relativity
Rydberg formula

A formula which describes all of the characteristics of hydrogen'sspectrum, including the Balmer, Lyman, Paschen, Brackett, andPfund series.
Schroedinger's cat (E. Schroedinger; 1935)

A thought experiment designed to illustrate the counterintuitiveand strange notions of reality that come along with quantummechanics.

A cat is sealed inside a closed box; the cat has ample air,food, and water to survive an extended period. This box isdesigned so that no information (i.e., sight, sound, etc.) canpass into or out of the box -- the cat is totally cut off fromyour observations. Also inside the box with the poor kitty(apparently Schroedinger was not too fond of felines) is a phialof a gaseous poison, and an automatic hammer to break it, floodingthe box and killing the cat. The hammer is hooked up to a Geigercounter; this counter is monitoring a radioactive sample and isdesigned to trigger the hammer -- killing the cat -- should aradioactive decay be detected. The sample is chosen so thatafter, say, one hour, there stands a fifty-fifty chance of a decayoccurring.

The question is, what is the state of the cat after that onehour has elapsed? The intuitive answer is that the cat is eitheralive or dead, but you don't know which until you look. But it is one of them. Quantum mechanics, on the other hands, saysthat the wavefunction describing the cat is in a superposition ofstates: the cat is, in fact, fifty per cent alive and fifty percent dead; it is both. Not until one looks and "collapses thewavefunction" is the Universe forced to choose either a live cator a dead cat and not something in between.

This indicates that observation also seems to be an importantpart of the scientific process -- quite a departure from theabsolutely objective, deterministic way things used to be withNewton.
Schwarzchild radius

The radius that a spherical mass must be compressed to in order totransform it into a black hole; that is, the radius of compressionwhere the escape velocity at the surface would reach lightspeed.
Snell's law; law of refraction

A relation which relates the change in incidence angle of awavefront due to refraction between two different media.
Speed of light in vacuo

One of the postulates of A. Einstein's special theory ofrelativity, which puts forth that the speed of light in vacuum --often written c, and which has the value 299 792 458 m/s -- ismeasured as the same speed to all observers, regardless of theirrelative motion. That is, if I'm travelling at 0.9 c away fromyou, and fire a beam of light in that direction, both you and Iwill independently measure the speed of that beam as c. One of the results of this postulate (one of the predictionsof special relativity is that no massive particle can beaccelerated to (or beyond) lightspeed, and thus the speed of lightalso represents the ultimate cosmic speed limit.
Only masslessparticles (photons, gravitons, and possibly neutrinos, should theyindeed prove to be massless) travel at lightspeed, and all otherparticles must travel at slower speeds.
Spin-orbit effect

An effect that causes atomic energy levels to be split becauseelectrons have intrinsic angular momentum (spin) in addition totheir extrinsic orbital angular momentum.
Static limit

The distance from a rotating black hole where no observer canpossibly remain at rest (with respect to the distant stars)because of inertial frame dragging.
Stefan-Boltzmann constant; sigma (Stefan, L. Boltzmann)

The constant of proportionality present in the Stefan-Boltzmannlaw. It is equal to

Stefan-Boltzmann law (Stefan, L. Boltzmann)

The radiated power (rate of emission of electromagnetic energy) ofa hot body is proportional to the emissivity, an efficiencyrating, the radiating surface area, and the fourth power of thethermodynamic temperature. The constant of proportionality is theStefan-Boltzmann constant.
Stern-Gerlach experiment (O. Stern, W. Gerlach; 1922)

An experiment that demonstrates the features of spin (intrinsicangular momentum) as a distinct entity apart from orbital angularmomentum.
Superconductivity

The phenomena by which, at sufficiently low temperatures, aconductor can conduct charge with zero resistance.
Superfluidity

The phenomena by which, at sufficiently low temperatures, a fluidcan flow with zero viscosity.
Superposition principle of forces

The net force on a body is equal to the sum of the forcesimpressed upon it.
Superposition principle of states

The resultant quantum mechnical wavefunction due to two or moreindividual wavefunctions is the sum of the individualwavefunctions.
Superposition principle of waves

The resultant wave function due to two or more individual wavefunctions is the sum of the individual wave functions.
Thomson experiment; Kelvin effect (Sir W. Thomson [later Lord Kelvin])

When an electric current flows through a conductor whose ends aremaintained at different temperatures, heat is released at a rateapproximately proportional to the product of the current and thetemperature gradient.
Twin paradox

One of the most famous "paradoxes" in history, predicted by
A.Einstein's special theory of relativity. Take two twins, born onthe same date on Earth. One, Albert, leaves home for a triparound the Universe at very high speeds (very close to that oflight), while the other, Henrik, stays at home at rests. Specialrelativity predicts that when Albert returns, he will find himselfmuch younger than Henrik. That is actually not the paradox. The paradox stems fromattempting to naively analyze the situation to figure out why.From Henrik's point of view (and from everyone else on Earth),Albert seems to speed off for a long time, linger around, and thenreturn. Thus he should be the younger one, which is what we see.But from Albert's point of view, it's Henrik (and the whole of the
Earth) that are travelling, not he. According to specialrelativity, if
Henrik is moving relative to Albert, then Albertshould measure his clock as ticking slower -- and thus Henrik isthe one who should be younger. But this is not what happens.

So what's wrong with our analysis? The key point here is thatthe symmetry was broken. Albert did something that Henrik didnot -- Albert accelerated in turning around. Henrik did noaccelerating, as he and all the other people on the Earth canattest to (neglecting gravity). So Albert broke the symmetry, andwhen he returns, he is the younger one.
Ultraviolet catastrophe

A shortcoming of the Rayleigh-Jeans formula, which attempted todescribe the radiancy of a blackbody at various frequencies of theelectromagnetic spectrum. It was clearly wrong because as thefrequency increased, the radiancy increased without bound;something quite not observed; this was dubbed the "ultravioletcatastrophe." It was later reconciled and explained by theintroduction of Planck's radiation law.
Universal constant of gravitation; G

The constant of proportionality in Newton's law of universalgravitation and which plays an analogous role in A. Einstein'sgeneral relativity. It is equal to 6.664.10-11 N.m2/kg2.
Van der Waals force (J.D. van der Waals)

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