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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Physical crystallography before X-rays | 3/5 | https://en.wikipedia.org/wiki/Physical_crystallography_before_X-rays | reference | science, encyclopedia | 2026-05-05T16:17:31.939542+00:00 | kb-cron |
Luminescence is the non-thermal emission of visible light by a substance; an example is the emission of visible light by minerals in response to irradiation by ultraviolet light. The term luminescence was first used by Eilhard Wiedemann in 1888; he stated that luminescence was separate from thermal radiation, and he distinguished six different forms of luminescence according to their excitation. Fluorescence is luminescence which occurs during the irradiation of a substance by electromagnetic radiation; fluorescent materials stop emitting light nearly immediately after the irradiation is halted, except in the case of certain materials exhibiting delayed fluorescence (e.g., TADF, TTA). The term fluorescence was coined by George Stokes in 1852, and was derived from the behavior of fluorite when exposed to ultraviolet light. Phosphorescence is long-lived luminescence; phosphorescent materials continue to emit light for some time after the radiation stops. In 1857 Edmond Becquerel invented the phosphoroscope, and in a detailed study of phosphorescence and fluorescence, showed that the duration of phosphorescence varies by substance, and that phosphorescence in solids is due to the presence of finely dispersed foreign substances. Becquerel suggested that fluorescence is simply phosphorescence of a very short duration. The most prominent phosphorescent material for 130 years was ZnS doped with Cu+, or later Co2+, ions. The material was discovered in 1866 by Théodore Sidot who succeeded in growing tiny ZnS crystals by a sublimation method. Some additional kinds of luminescence from crystals can arise from energy sources other than electromagnetic radiation.
Crystalloluminescence is the emission of light during crystal growth from solution, specifically during nucleation. The first observation was that of potassium sulfate which was reported by a number of researchers in the eighteenth century; other substances reported in the early literature which exhibit crystalloluminescence include strontium nitrate, cobalt sulfate, potassium hydrogen sulfate, sodium sulfate, and arsenious acid. In 1918 Harry Weiser summarised the research on crystalloluminescence up to that date. Neither the spectral distribution nor the excitation mechanisms of crystalloluminescence are understood. Triboluminescence is the generation of light when certain materials, for example quartz, are rubbed; fractoluminescence is the emission of light from the fracture of a crystal. The first recorded observation is attributed to Francis Bacon when he recorded in his 1620 Novum Organum that sugar sparkles when broken or scraped in the dark. The scientist Robert Boyle also reported on some of his work on triboluminescence in 1664. In 1677 Henry Oldenburg described the luminescence of fluorite, CaF2, on heating; this is termed thermoluminescence. In 1830 Thomas Pearsall observed that colourless fluorite could be coloured by discharging sparks from a Leyden jar held against it. In 1881 luminescence excited by cathode rays was described by William Crookes, and in 1885 Edmond Becquerel found that when crystals were bombarded by cathode rays they became coloured and also emitted light; this has been termed cathodoluminescence. In 1894 Eugen Goldstein showed that ultraviolet light has the same effect as cathode rays.
=== Reflection from opaque materials ===
The study of the optical properties of opaque substances has been closely linked with the development of suitable microscopes. The first instrument adapted to reflected light was the Lieberkühn reflector attributed to Johann Nathanael Lieberkühn. The use of polished and etched surfaces for this type of study was introduced by Jöns Jacob Berzelius in 1813. A theory of the light reflected from metals was put forward by Augustin-Louis Cauchy in 1848. In 1858 Henry Clifton Sorby established the technique of cutting minerals and crystals into thin sections for examination under the polarizing microscope. In 1864 Sorby studied the microscopical structure of minerals from meteorites. In 1888 Paul Drude published work on reflection from antimony sulfide.
=== Infrared optics === Heinrich Rubens measured the dependence of the refractive index of quartz on wavelength, and found absorption in particular infrared wavelength ranges. By 1896 Rubens saw these bands as a potential filter that would allow him to separate out an almost monochromatic beam from the broad range of infrared radiation that his sources produced. In 1897 Rubens and his student Ernest Fox Nichols studied the reststrahlen (residual rays) obtained when infrared rays of appropriate wavelength are reflected from the surfaces of crystals.
== Effect of electricity and magnetism ==
=== Electrical conduction === The first observations on the variation of electrical conductivity with direction in a crystal (anisotropy) were made by Henri Hureau de Sénarmont in 1850 on 36 different substances. The results showed a correlation between the axes of symmetry and the directions of maximum or minimum conductivity. In 1855 Carlo Matteucci performed experiments on bismuth. In 1888, Helge Bäckström performed electrical conduction measurements on hematite, another crystal of rhombohedral symmetry. Electrical conductivity in a crystal is now defined as a second rank symmetric tensor relating two vectors:
J
i
=
σ
i
j
E
j
,
{\displaystyle \mathbf {J} _{i}={\boldsymbol {\sigma }}_{ij}\mathbf {E} _{j},}
where
J
i
{\displaystyle \mathbf {J} _{i}}
is the current density,
σ
i
j
{\displaystyle {\boldsymbol {\sigma }}_{ij}}
is the electrical conductivity tensor, and
E
j
{\displaystyle \mathbf {E} _{j}}
is the electric field intensity.
=== Magnetic properties ===