Nuclear

Part 11 - Radiation

In 1814, Joseph von Fraunhofer invented the first spectroscope using a prism, diffraction slit and telescope to analyze light from the sun.

In 1859, Robert Bunsen and Gustav Kirchhoff heated various materials and used a spectroscope to analyze the incandescent light emitted. They identified the characteristic spectral lines of lithium, potassium and sodium and proved that each element had a unique spectra. In the process, they detected spectral emission lines of an undiscovered chemical element, which was later found to be caesium. A year later Bunsen discovered rubidium using the same technique. Spectroscopy was later used by metallurgists to determine the percentage of impurities and alloying elements in various materials like steel, aluminum and other products. I was invaluable instrument for analyzing elements in the outer layers of the Sun and other stars.

By 1900, after three years studying the problem, Max Planck proposed that radiation could be produced only at certain energies determined by a very small 'universal constant' (h) (Plank's constant) multiplied by the frequency (v).

Einstein suggested, in 1905, that the energy of electrons, knocked out of certain metals by light (the photo-electric effect), depended not on the brightness but on the colour (on its frequency). He suggested that light did not travel in waves, as had been proven by many experiments, but that it travelled in 'energy quanta' (now called photons). 

Photons had energy which depended entirely on the frequency and this could be transferred to electrons. (A brighter light of the same frequency merely produced more electrons). The idea of photons was not widely accepted until S. N. Bose derived the Planck spectrum in 1924. Niels Bohr considered the atomic model which had been proposed by Ernest Rutherford (a miniature solar system with a nucleus surrounded by orbiting electrons) to be unstable. He thought that the electrons could only occupy what he called stationary states so they would not emit light or spiral into the nucleus as they lost energy.

Hendrik Lorentz shared the 1902 Nobel Prize in Physics with Pieter Zeeman for the discovery and theoretical explanation of the Zeeman effect. He also derived the transformation equations underpinning Albert Einstein's theory of special relativity.

Jules Henri Poincaré proposed the principle of relativity in 1904 and recorded the remaining relativistic velocity transformations in a letter to Hendrik Lorentz in 1905, an important step in the formulation of the theory of special relativity. He also proposed gravitational waves and laid the foundations of modern chaos theory. 

Inspired by Maxwell's theory of electromagnetism, Einstein used these unexpected ideas to formulated the theory of special relativity in 1905. This derived the Lorentz transformation, length contraction and time dilation. . . . Space dimensions and time were no longer constants. 

Einstein's theory of relativity, best known for his mass–energy equivalence formula, E = mc2 (Energy = mass multiplied by the velocity of light squared), is one of the foundations of modern physics.

His discovery of the law of the photoelectric effect, an essential step in the evolution of quantum mechanics, was another indispensable contribution to physics. Einstein also extended the principle of relativity to gravitational fields and in 1916 he published a paper on general relativity and later developed explanations of particle theory and the motion of molecules.

Spectroscopy was well established by 1912. Johann Balmer in 1885 had derived a formula which predicted the spectral lines emitted by hydrogen and Johannes Rydberg developed this into a formula which worked for all spectra. As soon as Bohr saw Balmer's formula, he realized that the lines could be explained by electrons changing 'orbits'.

Add heat or other forms of energy to an object and the electrons would move to higher energy orbits, farther away from the nucleus. Remove the heat source and eventually the electrons would fall back to their ground state by radiating a photon of a particular energy and therefore frequency.

(If you heat a cast iron pan, it will first radiate heat at the lower (invisible infra-red) frequencies but as it gets hotter it will glow red and then white hot indicating that the electrons are jumping to even higher orbits and radiating visible radiation as they fall back to a lower orbit, or the ground state. Blacksmiths have used this phenomenon for thousands of years to estimate the temperature as they harden and temper steel).

A spectroscope will show lines according to the unique frequencies generated by the electrons falling back to lower orbits. This permits metallurgists to determine the percentage of elements in a sample of material. This technique is used to analyze elements in the surface of the sun (predominately hydrogen and helium) and other stars.

Bohr proved that the lines from the Balmer series coincided precisely with the energies of the photons emitted as the hydrogen electron fell down from orbit to orbit to the ground state.