Planck's law of black-body radiation

In 1893, German physicist Wilhelm Carl Wien advanced the black body research by developing relationship between the wavelength of peak emission of radiant energy by a black body and its absolute temperature (T).

A black body is a system that absorbs all the radiation that falls on it. For instance, a matte black body surface or a large cavity with a small hole in its wall is a black body and emits black body radiation.

In 1859, G.R Kirchhoff verified that the ratio so the emissive power to the absorptivity at a given temperature T is the same for all bodies, irrespective of the material of which the body is made.

In 1900, Lord Rayleigh proposed a new black body radiation law which was later modified by J. Jeans in 1905 and is now known as the Rayleigh-Jeans formula.

In his investigation, to find a relation between the radiation emitted by a black body as a function of temperature and wavelength, Max Planck (1858-1947) developed the now famous equation named after him.

He reported his findings, which were based on his experimental work on October 19, 1900. Planck's law describes the electromagnetic radiation emitted by a black body in thermal equilibrium at a definite temperature. It describes the distribution of energy radiated by a black body. It is a pioneering result of modern physics and quantum theory.

His efforts laid the foundation of the quantum theory, for which he received the Nobel Prize in 1918.
Planck's law of black-body radiation

History of X-ray

X-rays were discovered in 1895 by Wilhelm Conrad Rontgen at the University of Wurzburg, Bavaria. 

He noticed that some crystals of barium platinocyanide, near a discharge tube completely enclosed in black pepper, became luminescent when the discharge occurred. 

By examining the shadows cast by the rays, Rontgen traced the origin of the rays to the walls of the discharge tube. 

In 1896, Campbell-Swinton introduced a definite target (platinum) for the cathode rays to hit; this target was called the anticathode. 

For his work x-rays, Rontgen received the first Nobel prize in physics, in 1901. It was the first of six to be awarded in the field of x-rays by 1927. 

The obvious similarities with the light led to the crucial tests of established wave optics: polarization, diffraction, reflection and refraction. 

With limited experimental facilities, Rontgen and his contemporaries could find no evidence of any of these; hence, the designation “x” (unknown) of the rays, generated by the stoppage of anode targets of the cathode rays, identified by Thompson in 1897 as electrons. 

The nature of x-rays was the subject of much controversy. In 1906, Barkla found evidence in scattering experiments that x-rays could be polarized and must therefore by waves, but W.H Bragg’s studies of the produced ionization indicated that they were corpuscular. 

The essential wave nature of x-rays was established in 1912 by Laue, Friedrich, and Knipping, who showed that x-rays could be diffracted by a crystals (copper sulfate pentahydrate) that acted as a three dimensional diffraction grating.

W.H Bragg and W.L Bragg (father and son) found the law for the selective reflection of x-rays. In 1908, Barkla and Sadler deduced, by scattering experiments, that x-rays contained components characteristics of the material of the target; they called these component K and L radiations.
History of X-ray