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Tag archives: waves

How Charles Wheatstone got to see polarization

By Matin Durrani

Things seem to have quietened down a bit following last month’s announcement by astronomers in the BICEP2 collaboration that they had obtained the first evidence of cosmic inflation – the period of rapid expansion in the first fraction of a second after the Big Bang. As you’ll know if you’ve been keeping up, the evidence was obtained by searching for certain “B-mode” polarizations in the cosmic microwave background, which are related to primordial gravitational waves that are thought to have abounded in the early universe. These polarizations differ from “E-mode” polarization, which describes how the magnitude of polarization varies across the CMB.

But never mind your fancy B-modes and E-modes, how well do you understand the concept of polarization in the first place? Intriguingly, in the late 1840s Sir Charles Wheatstone, who was then professor of experimental philosophy at the University of London, decided to create a mechanical device to explain the principles of the concept – several decades before James Clerk Maxwell’s theory of the electromagnetic nature of light.

The video above shows a rare surviving example of one of these “Wheatstone Wave Machines”, which has been restored to working order by Robert Whitworth and colleagues at the University of Birmingham in the UK as part of their collection of historic physics instruments. Wheatstone designed the machine to visualize the wave nature of light and offer what Whitworth calls “a vivid insight into the theoretical concepts of wave motion”. At the time, there were other devices that showed the behaviour of travelling plane waves, but Wheatstone’s was different in that it was the first to demonstrate circularly polarized light.

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Reawakening the Kelvin wake

Wake pattern

Wake angles: the black dashed line shows the outer edge of the wake and is at the angle predicted by Lord Kelvin. The red line shows the maximum amplitude of the wake and is at the angle predicted by Marc Rabaud and Frédéric Moisy. (Courtesy: A Darmon, M Benzaquen, E Raphaël)

By Hamish Johnston

Loyal readers may recall that earlier this year we published a news article entitled “Physicists rethink celebrated Kelvin wake pattern for ships” that reported on work done by two French physicists. While looking at Google Earth images of ships moving through the sea, Marc Rabaud and Frédéric Moisy noticed that some of the wakes did not conform to a prediction made years ago by Lord Kelvin, the renowned Victorian physicist, engineer and entrepreneur.

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