Michelson-Morley experiment (A.A. Michelson, E.W. Morley; 1887)

Possibly the most famous null-experiment of all time, designed toverify the existence of the proposed "lumeniferous aether" throughwhich light waves were thought to propagate. Since the Earthmoves through this aether, a lightbeam fired in the Earth'sdirection of motion would lag behind one fired sideways, where noaether effect would be present. This difference could be detectedwith the use of an interferometer.

The experiment showed absolutely no aether shift whatsoever,where one should have been quite detectable. Thus the aetherconcept was discredited as was the constancy of the speed oflight.

Millikan oil drop experiment (R.A. Millikan)

A famous experiment designed to measure the electronic charge.Drops of oil were carried past a uniform electric field betweencharged plates. After charging the drop with x-rays, he adjustedthe electric field between the plates so that the oil drop wasexactly balanced against the force of gravity. Then the charge onthe drop would be known. Millikan did this repeatedly and foundthat all the charges he measured came in integer multiples only ofa certain smallest value, which is the charge on the electron.

Newton's law of universal gravitation (Sir I. Newton)

Two bodies attract each other with equal and opposite forces; themagnitude of this force is proportional to the product of the twomasses and is also proportional to the inverse square of thedistance between the centers of mass of the two bodies.

Newton's laws of motion (Sir I. Newton)

Newton's first law of motion. A body continues in its state of rest or of uniform motion unless it is acted upon by an external force.

Newton's second law of motion. For an unbalanced force acting on a body, the acceleration produces is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.

Newton's third law of motion. In a system where no external forces are present, every action is always opposed by an equal and opposite reaction.

Ohm's law (G. Ohm; 1827)

The ratio of the potential difference between the ends of aconductor to the current flowing through it is constant; theconstant of proportionality is called the resistance, and isdifferent for different materials.

Olbers' paradox (H. Olbers; 1826)

If the Universe is infinite, uniform, and unchanging then theentire sky at night would be bright -- about as bright as the Sun.The further you looked out into space, the more stars there wouldbe, and thus in any direction in which you looked your line-of-sight would eventually impinge upon a star. The paradox isresolved by the Big Bang theory, which puts forth that theUniverse is not infinite, non-uniform, and changing.

Pascal's principle

Pressure applied to an enclosed imcompressible static fluid istransmitted undiminished to all parts of the fluid.

Paschen series

The series which describes the emission spectrum of hydrogen whenthe electron is jumping to the third orbital. All of the linesare in the infrared portion of the spectrum.

Pauli exclusion principle (W. Pauli; 1925)

No two identical fermions in a system, such as electrons in anatom, can have an identical set of quantum numbers.

Peltier effect (J.C.A. Peltier; 1834)

The change in temperature produced at a junction between twodissimilar metals or semiconductors when an electric currentpasses through the junction.

permeability of free space; magnetic constant; m ₀

The ratio of the magnetic flux density in a substance to theexternal field strength for vacuum. It is equal to 4 p ^. 10^-7 H/m.

permittivity of free space; electric constant; e₀

The ratio of the electric displacement to the intensity of theelectric field producing it in vacuum. It is equal to 8.854 ^.10^-12 F/m.

Pfund series

The series which describes the emission spectrum of hydrogen whenthe electron is jumping to the fifth orbital. All of the linesare in the infrared portion of the spectrum.

Photoelectric effect

An effect explained by A. Einstein that demonstrate that lightseems to be made up of particles, or photons. Light can exciteelectrons (called photoelectrons) to be ejected from a metal.Light with a frequency below a certain threshold, at anyintensity, will not cause any photoelectrons to be emitted fromthe metal. Above that frequency, photoelectrons are emitted inproportion to the intensity of incident light. The reason is that a photon has energy in proportion to itswavelength, and the constant of proportionality is Planck'sconstant. Below a certain frequency -- and thus below a certainenergy -- the incident photons do not have enough energy to knockthe photoelectrons out of the metal. Above that threshold energy,called the workfunction, photons will knock the photoelectrons outof the metal, in proportion to the number of photons (theintensity of the light). At higher frequencies and energies, thephotoelectrons ejected obtain a kinetic energy corresponding tothe difference between the photon's energy and the workfunction.

Planck constant; h

The fundamental constant equal to the ratio of the energy of aquantum of energy to its frequency. It is the quantum of action.It has the value 6.626196 ^.10^-34 J ^.s.

Planck's radiation law

A law which more accurately described blackbody radiation becauseit assumed that electromagnetic radiation is quantized.