electromagnetic radiation

the origin of the Universe

electromagnetic radiation & optics - a historical account

were fascinated by the concept of matter & how it was formed & structured.

Zeno (470-460BC) “all matter was like a huge jelly that filled all space”

in 500AD, had developed a complex theory of atomism which included the notion of atomic size & that atoms were 10^-9 cm in size which is remarkably close to modern measurements!!

medieval alchemists believed that it would be possible to transmute common base  metals into noble metals such as gold & silver, unfortunately for them they failed

by the time of Robert Boyle in 2nd half of the 17thC, when the foundations of modern chemistry were being laid, it became increasingly established in belief that changing an element from one metal to another was impossible.

1658: Gassendi revived the interest in atomic theory of the ancient Greeks & discussed the possible applications of atomic theory as well as coining the term 'molecule' for groups of atoms.

Boyle & Newton applied atomic theory to such scientific problems as the nature of sound & light & the principles of motion.

work on atomic weights & combinations set the scene for Dalton to start the development of the modern atomic theory.

1808: Gay-Lussac established that atoms combine in different ratios

1811: Avogadro suggested that molecules or clusters of atoms may be the smallest units of many elements & that equal volumes contained equal numbers of molecules (these were ignored until re-stated in 1858 by Cannizzaro).

Clerk Maxwell “atoms were and always had been, in all circumstances unchangeable”

1812: early expts in electrolysis leads to Berzelius assigned certain atoms a positive charge (eg. sodium) and others a negative charge (eg. chlorine) leading to an early theory of valency & the concept that atoms combine because they attract each other electrically.

relative atomic masses determined

1829: Dobereiner noticed that certain groups of 3 chemically similar elements had values which were in approx. arithmetic progression, forming Dobereiner's triads 

1838: Liebig idea of acids being compounds containing hydrogen replaceable by metals to form a “salt”

1839: Dumas proposed there were certain fundamental types of chemical compound & that any element or group of elements in these types could be replaced, equivalent for equivalent, by another element or group of elements. This theory was successful in organic chemistry & eventually developed the idea of homologous series.

1864: Newland's law of octaves - by arranging elements in ascending order of relative atomic mass & assigning the elements a series of ordinal numbers which he called atomic numbers, he noticed that similar elements had atomic numbers that differed by 7 or some multiple of seven.

1869: Mendeleef's law of periodicity, which essentially restated Newland's law, was supported by the periodic relationships noticed by Lothar Meyer in plotting atomic volumes ( relative atomic masses / densities ) of the elements against their relative atomic masses.

1895: X-rays were discovered by Rontgen forming the starting point of modern physics as well as revolutionising diagnostics in medicine.

1895: Ramsay showed that that a gas liberated from uranium had the same spectral characteristics as helium

1896 radioactivity of uranium was discovered by Becquerel, then in 1898, Curie discovered radium

Rutherford discovered alpha & beta rays and by 1899, the electron - the 1st sub-atomic particle - had been discovered.

1897: J.J.Thomson discovered that atoms were actually made of electrons, suggesting that atoms could be divided.

these, along with research into the photo-electric effect in 1898, were to set the scene for Rutherford to change our perception of matter with his vision of the atom & its structure, and for Einstein to change our perception of space and time.

1903: Rutherford renames “long penetrating rays” as gamma rays but their nature was uncertain

1903: Rutherford finally shows he can deflect alpha rays with both an electric field & a magnetic field & showed that the ratio of its charge to mass was approx. one half that for the hydrogen atom

1903: Soddy & Ramsay show that that a gas liberated from radium had the same spectral characteristics as helium

1903: Lenard using a cathode ray tube shows that an atom had an open structure & was mostly empty space

1904: Rutherford shows that the charge of an alpha particle is twice that of an electron & thus its mass must be 4 x that of hydrogen atom, therefore an alpha particle was probably an helium atom which was expelled during the process of disintegration

1905: Rutherford discovers that thorium's radioactivity decays with a half-life ⇒ “disintegration theory”

1908: Rutherford proves that alpha particles are helium atoms and working with Geiger, use the scintillation method for measuring the scattering when a narrow beam of alpha rays passes through a thin sheet of metal 

1909: Geiger & Marsden complete their scattering expts which surprisingly showed that occasionally, an alpha particle travelling at 10,000 miles/sec is bounced back, which suggested to Rutherford that atoms may have a small dense nucleus from which the particles bounced if hit directly.

1911: Rutherford's atom with a nucleus surrounded by electricity - his view of the atom as having a nucleus containing alpha particles, so well protected from, so inaccessible to, ordinary physical & chemical action, that it cannot be broken up by ordinary chemical or physical forces. The nucleus had a charge of magnitude Ne, where e = unit electronic charge & N is a whole number, and was surrounded by a sphere of electricity of the opposite kind. He could not say if the nucleus was positive or negative as either would account for the scatter paths of alpha rays.

1912: von Laue proves the wave nature of X-rays by passing them through a crystal & Moseley showed that different elements produce different X-ray spectra & that the square root of the characteristic frequency increased by a constant amount as one passed from one element to the next when the elements were arranged in order of atomic weight, but that it was the atomic number itself that correlated, not the weight. To Moseley, this proved that there is a fundamental quantity in an atom which increases by regular steps as we pass from one element to the next. This quantity can only be the charge on the central positive nucleus. Thus, the N in Rutherfords Ne nuclear charge was equivalent to the atomic number. He then predicted any missing elements could be identified using this & indeed 4 gaps were found & later elements were discovered to fill these gaps.

1912: Wilson devises his cloud chamber for photographing tracks of alpha particles

1913: after working with Rutherford, Geiger invents the Geiger counter

1913: discovery of isotopes - elements existing having identical chemical properties but different masses - suggested that nuclei have the same charge but different masses

1914: Rutherford and Andrade using X-ray crystal analysis methods, showed that gamma rays were of the same nature as X-rays but of shorter wave length

Rutherford shows that that particles which alphas knocked out of bombarded nitrogen were hydrogen nuclei to which he gave a special name, proton. This presumably led to Bronsted & Lowry in 1923 formulating their concept of acids & bases as being proton donors & proton acceptors respectively.

1920: Rutherford in his 2nd Bakerian lecture, predicts the existence of heavy hydrogen (deuterium - discovered in 1931) and also light helium, but most remarkable of all was his anticipation of the existence of a particle with zero nuclear charge, the neutron which was discovered in 1932 by Chadwick

1925: Uhlenbeck & Goudsmit postulate electron spin to account for the splitting of many single spectral lines into double lines when examined under a spectroscope of high resolving power.

Pauli exclusion principle - if two electrons occupy the same orbital, they must have different spins thus no orbital can contain more than 2 electrons.

1930: Ernest Lawrence invents the cyclotron, a machine to accelerate a particle to energy of tens of MeV.

in the 1920's, Heisenberg, Dirac & Schrodinger developed a new picture of reality called quantum mechanics

no longer did tiny particles have a definite position & speed but introduced the Uncertainty Principle which stated that the more is known about the position of a particle, the less can be known about its speed & vice versa

these became the foundation of modern developments in chemistry, molecular biology & electronics.

1924 de Broglie suggested that moving electrons had waves of definite wavelength which was demonstrated in 1927 and their wavelength = h/mv where h is Planck's constant, m is mass of electron & v is the velocity of the electron & if this is written as momentum x wavelength = h & thus relates the particle-like aspect of an electron ie. its momentum, to the wave-like aspect, ie. wavelength.

1927, Schrodinger postulated based on his intuition without proof, that the wave pattern of an electron could be expressed as an equation relating a wave function, the total energy & potential energy of the system, mass of the electron, Planck's constant & the coordinates of the system.

1927, Heisenberg put forward the Uncertainty Principle as a long wavelength can be measured with greater fractional accuracy than a short one, thus according to de Broglie's equation, a particle with small momentum has a correspondingly large wavelength which can be measured with some accuracy but at the expense of a relatively inaccurate determination of the small momentum.

1934: Joliot-Curies found that by bombarding light elements such as boron, magnesium & aluminium with alpha particles, radioactive isotopes could be produced, some of which gave out positrons in the course of their rather rapid decay, a new feature.

Fermi showed a special effectiveness of neutron particles in penetrating heavy elements with large nuclear charges which otherwise repel alphas so strongly that they cannot get near enough to cause any disintegration. He was able to produce, out of 63 elements investigated, 37 new elements showing radioactive properties, among these were new elements produced from thorium & uranium, which were heavier than any natural elements & they were radioactive in a different way by giving out beta particles (electrons) instead of alpha particles. He also showed that neutrons that had been slowed down by repeatedly hitting protons, were more effective in producing atomic transmutations than fast neutrons. 

1936: Rutherford predicts the possibility of producing energy on an industrial scale from nuclear transmutation.

1937: Rutherford dies.

1938: in response to Mussolini adopting  the Nazi code, Fermi leaves Italy to go to US

Einstein predicts the possibility of releasing enormous amounts of energy by splitting the atom & thus nuclear explosions.

1940: Seaborg & McMillan discover plutonium;

2 December 1942: Fermi et al at Chicago demonstrated that the chain reaction created by 'splitting the atom' could be contained & was thus self-sustaining. This was the vital experiment that ensured that all the previous work would eventually lead to the atomic bomb.

it took 2.5yrs, the labour of 500,000 people & $1.4b before the Manhattan Project as it was called, successfully converted Fermi's expt into a bomb. Much of the expense entailed taking quantities of raw uranium & producing adequate amounts of the rare but highly fissionable material U-235.

16 July 1945: at Alamogordo in New Mexico, the 1st atom bomb was successfully detonated.

6 August 1945, an atom bomb using uranium was dropped on Hiroshima, and 3 days later an atom bomb using plutonium was dropped on Nagasaki, the resulting explosions killing 120,000 people but resulted in an end to the war with Japan.

nuclear energy may also be produced by nuclear fusion which is the fusion of small nuclei into larger nuclei with loss of total mass in the system & thus release of energy. Nuclear fusion is the process that occurs on the sun as it converts hydrogen atoms to helium, but as this requires temperatures found in stars whilst the reactants must be confined within walls which wont melt (perhaps a magnetic containment field), it has yet to be made possible by man. The initial steps would be to heat the hydrogen atoms sufficiently to strip them of electrons & thus produce a “fourth state of matter” - a fully ionised gas or plasma.

the hydrogen bomb is large scale fusion of deuterium using a fission bomb in the centre to produce the high temperatures required for fusion.

1939: Sidgwick-Powell rule - the approximately spherical charge clouds within a particular electronic shell will stay as far away from each other as possible 

1951: Bethe & van Vleck introduce crystal or ligand field theory, initially to ionic crystals, but Orgel developed its more general chemical aspects.

1957: Gillespie & Nyholm fully develop use of the Sidgwick-Powell rule to predict molecular shapes

matter consists of fundamental matter particles quarks & leptons (eg. electron, muon, tauon & neutrinos) with 6 flavours of each

 

an extension of quantum physics using strings (one-dimensional extended objects that vibrate) to represent the “particles” form the super-string theories of the mid-1980's

see the origin of the Universe

1928: a young physicist named Paul Dirac solved the problem of combining quantum theory with special relativity by creating a strange mathematical equation but this also in some way, predicted the existence of an antiworld identical to ours but made out of antimatter. Dirac speculated on the existence of a completely new Universe made out of antimatter!

1930: the invention of the cyclotron - when a particle is accelerated to the speed of light then is rapidly stopped, it gives off energy in the form of incredible heat which in turn creates a particle and an anti-particle, these if they collide, will annihilate each other and give off energy in the form of light.

1932: Carl Anderson discovers the anti-electron which he called a positron which was confirmed by Blackett in 1933.

1954: Lawrence invents the Bevatron, a particle accelerator optimised to produce anti-protons by colliding two protons at 6.2GeV. The anti-proton was discovered in 1955 by Segre & his group.

1960: the anti-neutron is discovered.

1965: an anti-deuteron (a nucleus of antimatter made out of an anti-proton & anti-neutron) was created at Cern.

1980's: NASA consider antimatter as a propulsion as it is the perfect fuel with all its matter converted into energy - 1kg converted to energy equates to 25,000,000,000 kWh energy - 1g would supply a medium sized town for 1 day. But antimatter proved to be very difficult to create & requires enormous amounts of energy!

1995: Cern's unique Low Energy Antiproton Ring (LEAR) that slows down anti-protons creates 9 high energy anti-hydrogen atoms. CERN can produce 50 millions antiprotons in each cycle (about once a minute), that allows them to make a few hundred antihydrogen atoms (ie. a billionth of a gram per year). Cern thus decided to create the 1st “self-contained antiproton factory” - the Antiproton Decelerator (AD) which produces the low energy antiprotons needed to make antimatter.

 2002: Cern create 50,000 low energy anti-hydrogen atoms.