X-Rays discovery

Wilhelm Roentgen was professor of physics at university of Wurzburg, Germany when he discovered X-rays in 1985. The discovery was entirely serendipitous; Roentgen was merely studying a beam of electrons in a highly evacuated glass vessel. When the electrons, moving at great speed slammed into glass wall, they produced a very high penetrating radiation - a wholly unexpected occurrence. Roentgen first noticed the radiation when it caused a paper coated with Barium Platino-cyanide to glow. The chemical compound was a standard detector of UV light which causes the chemical to fluorescence i.e. to emit visible light after it has absorbed UV light. But Roentgen's evacuated vessel was tightly covered with black cardboard and so no UV light could emerge from it. The glow must be some other kind of radiation.

When he announced the discovery of the new radiation, Roentgen wrote:

"I posesss, for instance, photographs of ............the shadow of bones of hand, the shadow of a covered wire enclosed in a box.........."

Earlier in the paper, he noted that "the darker shadow of bones is seen with in the slightly dark shadow image of hand itself.

The new radiation quickly became a diagnostic tool in hospitals all over the world. Roentgen could not determine what the rays are made of and thus rays are named as X-rays.       

PHYSICS DICTIONARY

A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z

             <Prev>                   

Zeeman Effect

In a magnetic field, the energy of a particular atomic state depends on value ‘m’, the magnetic quantum number. A state of total quantum number ‘n’ breaks up into several sub-states when the atom in the magnetic field and the energies are slightly more or less than energy of state in the absence of magnetic field. This phenomenon leads to splitting of individual spectral lines when atoms radiate in magnetic field. The spacing of lines depends on magnitude of fields.  

Zero Point Energy

 Energy possessed by atoms or molecules even at absolute zero.

Zeroeth Law of Thermodynamics

This law was first enunciated by R H Fowler in 1831. According to this law, when two systems A and B are in thermodynamic equilibrium with another system C, then A & B will also be in thermal equilibrium. 

Zone Plate

The optical device which verifies rectilinear propagation of light approximately by wave theory. 

PHYSICS DICTIONARY

A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z

<Prev>                   <Next>

Year

Time taken by earth to complete one revolution around the sun.

Yield Point

 The maximum stress level at which plastic deformation begins.

Yielding

The phenomenon of transformation of elastic deformation to plastic deformation is called as yielding.

Young’s Modulus

The ratio of longitudinal stress to longitudinal strain with in elastic limit is called young’s modulus.

PHYSICS DICTIONARY

A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z

<Prev>                   <Next>

X-ray Astronomy

This branch of physics was born in 1962 when Italian–American astronomer Riccardo Giacconi discovered a cosmic x-ray source in the form of a compact star located in the constellation of Scorpius. For this discovery Giacconi received the 2002 Nobel Prize in Physics. X-ray astronomy is a relatively new branch of astronomy dealing with the study of x-ray emission from celestial objects, such as neutron stars, pulsars, and black holes. Since the X-rays emitted by celestial objects have relatively low energies of the order of a few KeV, they cannot penetrate through the Earth’s atmosphere to reach the surface of the Earth. Thus, to study these celestial rays, detectors must be taken above the Earth’s atmosphere. Methods used to achieve this involve mounting x-ray detectors on rockets, balloons, or satellites. The x-ray detectors used for this purpose are either special charge-coupled devices (CCDs) or micro calorimeters.

X – Ray Crystallography

 X- ray crystallography is a study of crystal structures through the use of X-ray diffraction techniques. X-rays are very suitable for this purpose because their wavelength in the 0.1 Ao (~ 100KeV) to 1 Ao (~10 KeV) range is of the order of typical crystalline lattice separations. An X-ray beam striking crystalline lattice is scattered by the spatial distribution of atomic electrons and the imaged diffraction pattern provides information on the atomic or molecular structure of the crystalline sample. In 1912, Max Von Laue established the wave nature of X rays and predicted that crystals exhibit diffraction phenomena. Soon, there after William H. Bragg and William L. Bragg analyzed the crystalline structure of sodium chloride, derived the Bragg relationship and laid the foundation for X- ray crystallography.

X Ray Spectroscopy

X ray spectroscopy is an analytical technique for determination of elemental composition of solid or liquid sample in many fields Such as material science, environmental science, geology, biology and forensic science. The technique is divided into three related categories. The most common of them is the X-ray absorption spectrometry (also called as X-ray fluorescence spectrometry). The other two are X-ray photoelectron spectrometry and auger spectrometry. All three techniques rely on creation of vacancies in atomic shells of the various elements in the sample under study as well as on an analysis of the effects that accompany the creation of vacancies.

X-rays

Electromagnetic radiation emitted by charged particles (usually electrons) in changing atomic energy levels.   

         

PHYSICS DICTIONARY

A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z

<Prev>                   <Next>

Wave Front

 The wave front of a wave is a surface at each point of which the vibrating particles are in same phase.

Wave Length

 Distance travelled by the wave in the time in which particle of medium completes one vibration.

                                              or

 Distance between two nearest particles in the same phase.

Wave Motion

It is a form of disturbance which travels through the medium due to repeated periodic motion of particles of medium about their mean positions. Wave motion, in general, refers to transfer of energy from one point to another point of medium. It is not matter that is propagated but it is only state motion of matter that is propagated. It is a form of dynamic condition that is propagated from one point to the other point in the medium.  

Wave

Wave is pattern and doesn’t have any physical existence. Wave motion comprises of patterns repeating at regular intervals and thus wave travels with uniform velocity.

Weidmann-Franz Law

The linear dependence of “ratio of thermal conductivity to electrical conductivity” on absolute temperature is known as Weidmann-Franz law. The ratio is directly proportional to absolute temperature.

 

Weight Percent (wt%)

Weight of a particular element relative to the total alloy weight.

Weld Decay

Intergranular corrosion that occurs in some welded stainless steels at regions adjacent to the weld.

Welding

 Process of permanently joining two similar metals by localized fusion by applying suitable heat and pressure is called as welding.

Wet Bulb Temperature

Wet-bulb temperature represents how much moisture the air can evaporate. This temperature is often measured with a common mercury thermometer that has the bulb covered with a water-moistened wick and with a known air velocity passing over the wick.

White Dwarf

They are stars which are fainter, possess small diameter, and are very dense compared to other stars of same mass. A white dwarf star is a mass of Helium at an extremely high temperature and under external compression. At its central temperature, Helium atoms are expected to be completely ionized and the star may be regarded as gas composed of Helium nuclei and electrons.

Wiedmann Franz Law

The ratio of thermal conductivity(K) to electrical conductivity σ is a constant (called Lorentz number) at a particular temperature for all the metals.

or

Weidmann–Franz discovered in 1853 that all good electrical conductors are also good thermal conductors. Ratio of K to σ is constant at a particular temperature for all the metals. Later Lorentz showed that this ratio is proportional to absolute temperature ‘T’.   K/σT= const=L; L is called as Lorentz number.

Wien’s Displacement Law

The wavelength of radiation emitted by a black body at any temperature is inversely proportional to absolute temperature of the body.

or

At any temperature, the energy of radiations of wavelength emitted by black body is directly proportional to fifth power of its absolute temperature.

Work

If the applied force on object leads to some displacement then it is said that work is done on object.

Work-Energy Theorem

The work done by the resultant net force acting on a body is equal to change in its kinetic energy.

 

Wrought Alloy

 A metal alloy that is relatively ductile and amenable to hot working or cold working during fabrication.