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Optical connections The supreme experimenter David Baker describes the optical experiments of Michael that led to his discovery of the magneto-optical or Faraday effect

ichael Faraday thing to do: ‘ and electricity are (1791-1867) must two great searching investigators of be considered the the molecular structure of bodies,’ pre-eminent name in he declared in a letter. Faraday the fields of electrical had also had some contact with and magnetic a young William Thomson (later research.M His importance as an Lord Kelvin), who was musing on experimental scientist in these subjects what effect a transparent is reflected in the fact that he is the (a poor conductor that can support only scientist to have been honoured an electrostatic field) might have on with two SI units of measurement polarised light. He referred Thomson being named after him (the to his own negative results of 1833, Faraday and the ). But, almost but was spurred into resuming his overshadowed by his seminal work investigations. Faraday’s diary for 30 on electromagnetism, are his lesser- August 1845 has a heading ‘Polarized known yet nevertheless important light and electrolytes’ under which contributions to experimental and he proposed to recommence the study theoretical . of the relationship between light and Faraday’s first experiments relating electricity. to light, in the 1830s, were an In the first experiments he passed investigation of the transmission of polarised light through various electricity by sparks. It followed from solutions being electrolysed. The lack work done by Charles Wheatstone of any change in the light confirmed in which an electric current was his 1833 results. Next he switched to passed along an extremely long (half solid media such as a silicate borate a mile) wire circuit containing three of lead glass that he had made years equidistant spark gaps in order to earlier. Still a nil result. Then he examine the nature and velocity of looked back to experiments he had electricity. Using a rotating mirror he carried out some months earlier on the was able to see that the middle spark be acknowledged as the theoretical Michael nature of magnetism and decided to was retarded with respect to the other foundation of the study of spark Faraday combine this work with his current two which flashed simultaneously. spectra. Faraday was using light experiments on light. Now, instead of It was Faraday who realised the mainly as a tool for his electrical electrolytes, he decided to try passing importance of this result for a theory investigations and, apart from some polarised light through various of electricity as being a probable link work in 1833 on the effect of passing transparent media positioned within between conduction and induction. polarised light through an electrolyte lines of magnetic force. He varied Wheatstone’s experimental which produced a nil result, his He set up electromagnets in five set-up and made several discoveries. optical studies came to a halt until different arrangements and initially Firstly, he replaced one of the 1845. used air as the medium, possibly wires connecting the electrodes in the as a control experiment. The state circuit with relatively poor conductors Light and electricity of polarisation of the light passed such as water or glass and found that What made Faraday return to through the magnetic fields was the retardation of the middle spark studies involving light? In the analysed with a Nicol prism. There increased. He then concentrated on ongoing debate about the nature was no effect observed with any of the effects of sparking electrodes of atoms, Faraday held to a ‘centre the arrangements with air or through in different gases. Here he found of force’ atomic theory in which various solid transparent media. that ‘the characters of the electric he assigned optical, magnetic and Then he tried the lead glass used spark in different gases vary,’ and other properties to the atmosphere in previous experiments. In one of considered the spark colour to be of forces surrounding the centre. the arrangements he found that the peculiar to each gas. Although he This, he felt, had implications for light passing through the glass in did not examine the spectra of the ‘the theory of light and the supposed the had been partly sparks created by different electrodes aether’. Using light to investigate the depolarised. He had discovered the and gases, Faraday’s work came to nature of matter seemed the obvious magneto-optical effect, now known

28 | Optician | 27.08.10 opticianonline.net Optical connections

as the Faraday effect. In his diary he concluded that electricity had and the magnetic properties he wrote, ‘Thus magnetic force and insufficient strength to render the of the sun’s coronal . light were proved to have relation to phenomenon visible. He also realised The supreme experimenter, Faraday each other. This fact will most likely that the ‘magnetic force does not act was the man of electromagnetism. He prove exceedingly fertile and of great on the ray of light directly (as witness was only one of many making optical value in the investigation of both non action in air etc), but through discoveries, but his contribution conditions of natural force’. This was the mediation of special matters’. to optical theory was important a then rare instance of an important In other words, magnetism affected nonetheless. ‘The world little knows optical discovery by experiment the behaviour of polarised light by how many of the thoughts and without recourse to the analytical influencing the matter through which theories which have passed through mathematical theory of light that was the light was passing: indeed, ‘a new the mind of the scientific investigator now becoming prevalent. Herschel magnetic force or mode of action on have been crushed in silence and and others had been searching matter’. secrecy by his own severe criticism unsuccessfully for this effect more The Faraday effect can be defined and adverse examination; that in the than 20 years earlier, so news of as the rotation of the plane of most successful instances not a tenth Faraday’s discovery was rapidly polarisation of linearly polarised of the suggestions, the hopes, the disseminated and acclaimed. light passed through a medium in wishes, the preliminary conclusions the direction of the lines of force of have been realized.’ So Faraday wrote. Further investigations an applied magnetic field. Having It’s fair to say that he realised more Faraday further investigated the provided the first evidence that light than most. ● conditions under which this new and electromagnetism are related, effect could be demonstrated. He developed in Acknowledgement found that (as recorded in his Diary), mathematical form the theoretical I would like to thank Professor Frank James, ‘when the polarized ray passes parallel basis for this. The Faraday effect has Professor of the History of Science and to the lines of magnetic induction, or some important modern applications Head of Collections at The Royal Institution, rather to the direction of the magnetic in measuring instruments. Examples and editor of The Correspondence of curves, that the glass manifests its include ‘spintronics’ which studies , for taking the time to point power of affecting the ray’. By the polarisation of spin in me in the direction of relevant material for using a stronger electromagnet he semiconductors; measurement of this article. was able to demonstrate his effect optical rotator power; and remote in other materials too. Having not sensing of magnetic fields, especially ● David Baker is an independent 9792 Ready Readers Advert v2 - no offer:Layout 1 20/5/10 14:19 Page 1 observed the effect with electricity, useful in for analysing optometrist Take a fresh look at our new fashionable ready readers... High quality ready readers and accessories - made by opticians for opticians.

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