The polarisation of the sky

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When light from the Sun or the Moon strikes Earth’s atmosphere it is scattered, sent in all directions by the atoms and molecules that make up the air. During this scattering process some of the light is polarised – instead of the electric and magnetic fields oscillating in many planes simultaneously, they oscillate in only one plane.

The polarisation of the light from the Sun or Moon is at right angles to the direction that the light is coming from; when the Sun or the Moon is very low in the sky (at sunset/sunrise or moonset/moonrise) the direction of polarisation is parallel to the horizon. The degree of polarisation depends on the angle between the light and the atmosphere, with the greatest degree of polarisation occuring when looking at 90° to the source.

Source: Christopher Kyba

The image on the left shows the degree of polarisation observed in a rural location. The image on the right shows the same section of the night sky, but observed from an urban location (the circular patterns in both images are caused by the movement of stars across the sky.) Because light pollution from streetlamps is not polarised, the effect of the streetlamps is to destroy the polarisation “data” that some animals use to navigate.

Source: C. C. M. Kyba, T. Ruhtz, J. Fischer and F. Hölker “Lunar skylight polarization signal polluted by urban lighting”, Journal of Geophysical Research 116 (2011). doi: 10.1029/2011JD016698.

The cost of coins

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It used to be the case that coins used as currency had value because of the material from which they were made: a solid gold coin weighing one ounce had the same value as one ounce of gold. The introduction of fiat currencies, issued by central banks, that only have value because a government decrees by law that they do, allowed governments to move away from this dependence on rare metals.

UK coins come in four “flavours”: the “copper” 1p and 2p coins, the “silver” 5p, 10p, 20p and 50p coins and the larger and heavier £1 and two-tone £2 coins. Before 1992 the “copper” 1p and 2p coins really did contain copper, being made from a bronze containing 97% copper, 2.5% zinc and 0.5% tin. With a mass of 3.56 and 7.12 grams respectively the pre-1992 coins contained 3.54 and 6.91 grams of copper; at the current bulk price and the current USD-GBP exchange rate this makes a 1p coin worth 1.66 pence and a 2p coin worth 3.23 pence (those links will take you to automatically calculated values using current values for spot price and exchange rate).

If you were able to buy pre-1992 1p and 2p coins in bulk for their face value you could make a profit by melting them down and selling the resulting copper as scrap (though this is illegal under the 1971 Coinage Act). This increasing price of copper in the early lead to a change from bronze to copper-plated steel and this makes it very easy to differentiate between the old and new 1p/2p coins: pre-1992 coins are not magnetic and post-1992 coins are.

The “silver” coins have previously been made from cupro-nickel, an alloy of copper and nickel in a 75:25 ratio. In terms of its raw metal, a 3.25 gram five pence coin, containing 2.44 grams of copper and 0.81 grams of nickel is currently worth 3.08 pence (1.14p for the copper and 1.94p for the nickel). A 6½ gram ten pence coin is worth 6.16p, a five gram twenty pence coin is worth 4.74p and an eight gram fifty pence coin is worth 7.58p.

From January 2012 the “silver” coins will be made from nickel-plated steel (making them magnetic), a change driven probably by the rising cost of both copper and nickel. (In May 2007, when the price of nickel was at a peak, a five pence coin was worth more than six pence as scrap copper and nickel).

The high value of the £1 coin (current value), composed of 70% copper, 24.5% zinc, and 5.5% nickel and the £2 coin, with a cupro-nickel core surrounded by a ring of 76% copper, 20% zinc, and 4% nickel, makes them unlikely to ever approach a situation in which they are worth more than their face value.

Technetium-99m generators

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Technetium-99m is a radioactive tracer that is used in twenty million medical diagnostic procedures per year. At least 31 radiopharmaceuticals based on Tc-99m are used for imaging and studying organs such as the brain, heart muscle, thyroid, lungs, liver, gallbladder and kidneys, as well as the skeleton and blood and for the investigation of tumours.

The ‘m’ in the name of technetium-99m indicates that it is metastable. Tc-99m is radioactive because one or more of the protons and neutrons in its nucleus is in an excited state. Tc-99m decays into Tc-99 with a half-life of six hours and this makes it particularly well suited to use in the body: after one day (four half-lives) only 6.3% of the initial Tc-99m remains. (It’s worth noting that the non-metastable technetium-99 is also radioactive, but with a half-life of 211000 years, it presents a very low risk.)

This short half-life also creates a problem: obtaining Tc-99m when required. Hospitals cannot run their own nuclear reactors and so they rely on technetium generators – machines that produce Tc-99m from the decay of its parent isotope molybdenum-99. Molybdenum-99 has a longer half-life (66 hours) and can therefore be transported to hospitals and still remain useful for up to a week.

Molybdenum-99 is produced in nuclear reactors by bombarding a highly enriched uranium target with neutrons, causing it to fission, forming Mo-99 (and many other isotopes) as it does. The vast majority of Mo-99 is produced by five nuclear reactors around the world that are specifically devoted to the production of nuclear isotopes for medicine: NRU in Canada, BR2 in Belgium, SAFARI-1 in South Africa, HFR Petten in the Netherlands and OSIRIS-1 in France.* Temporary shutdowns of NRU and HFR Petten in the 2000s led to a long-term shortage of Mo-99.

Once Mo-99 has been produced it is placed into a technetium generator and these generators are transported to hospitals. The technetium generators make use of the fact that molybdenum likes to bond with aluminium oxide (alumina) but technetium does not. The generators are “milked” by drawing a saline solution across an inner molybdenum/alumina capsule; during this elution process any technetium that has formed will be drawn away with the saline and can then be used in tests.

A cutaway model of a technetium generator.

The molybdenum/alumina sample is placed in the centre of the device, surrounded by shielding (painted red in this case). Saline is injected through one of the tubes at the top of the device and flows into a shielded container through the other tube, after having passed over the sample and “picked up” radioactive technetium-99m.

* Mo-99 is also produced in much smaller amounts from low-enriched uranium at the OPAL reactor in Australia and at other sites.

Plug wiring colour scheme

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UK plugs use brown insulation for the live wire, blue insulation for the neutral wire and green with yellow stripes insulation for the earth wire.

But why this particular combination of colours? The answer is deceptively simple: there is no type of colour blindness that will result in these wires becoming confused.

Above: how a UK plug looks to someone who is red-green colourblind.

Above: how a UK plug looks to someone who is blue-yellow colourblind.

One of the lesser-known safety features of a UK plug is the extra distance that the neutral wire has to travel when compared to the live wire. If someone pulls on the mains cable the live wire will disconnect first, making the plug safer.

Under the IEC 60446 standard only black, brown, red, orange, yellow, green, blue, violet, grey, white, pink and turquoise are acceptable colours for labelling wires. Countries must choose an appropriate selection of colours that eliminates the possibility of confusion.

IEC 60446 colours. From top to bottom: normal vision, deuteranopic vision, tritanopic vision.

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