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Flash Physics: Frogs see colour at night, new lens sharpens X-rays, PICO-60 puts new limits on dark-matter

01 Mar 2017 Hamish Johnston

Flash Physics is our daily pick of the latest need-to-know developments from the global physics community selected by Physics World's team of editors and reporters

Froggy vision: behavioural tests show amphibians see colour in the dark

Amphibians have colourful night-time vision

Frogs and toads can see colour when it is too dark for humans to see. Many animals have evolved to have impressive visual skills, yet understanding how and what they see has always been a challenge for scientists. A group from Lund University in Sweden and the University of Helsinki in Finland has therefore used behavioural studies to investigate the vision of frogs. Human eyes contain two types of visual cells in the retina – cones and rods. The cones allow us to see colour, but they stop working at night because they require a lot of light. At this point the rods take over and enable us to see in black and white under low-light conditions. Although this is the case for most vertebrates, frogs and toads are unique in that they have two spectrally different kinds of rod photoreceptors. Scientists have long hypothesized that these allow them to see in light levels that are too low for humans. While Carola Yovanovich and colleagues have shown this is true, they have also found that the amphibians can actually see colour in extreme darkness. The team performed three behavioural experiments to study how the frogs and toads used vision during different natural scenarios in near complete darkness. When searching for a mate, the animals stopped using colour information fairly early on in the process. Meanwhile, when hunting for food and escaping dens and passageways, they used all sensory information available to them including the ability to see colour in extremely dark surroundings. The study can be found in the Philosophical Transactions of the Royal Society B theme issue on “Vision in dim light”.

New lens sharpens X-ray beams

X-rays can now be focused with much greater precision using a new corrective lens developed by an international team of scientists. The component is designed for use on X-ray synchrotrons, which supply coherent beams of radiation for applications including condensed-matter physics, biology and chemistry. X-rays obey the same optical laws as light but because they have very short wavelengths and much higher energies it is very difficult to make lens systems that can steer and focus them. In particular, lens distortions of just a few hundred nanometres can have a detrimental effect on X-ray optics. While high-quality lenses made from beryllium are available, better optics could lead to improved measurements and even new types of experiments. Now, researchers at several universities in Germany and Sweden, and synchrotron labs in the UK, US and Germany, have carefully measured the distortions in a stack of beryllium lenses. They then used this information to create a corrective lens that was milled using a precision laser. Without the new lens, the beryllium stack could focus X-rays to a spot about 1600 nm in diameter. This was reduced to 250 nm by using the corrective lens. The research is described in Nature Communications.

PICO-60 puts new limits on dark-matter interactions

Physicists working on the PICO-60 detector in Canada have put a new upper limit on the strength of the spin-dependent interaction between the proton and hypothetical dark-matter particles called WIMPs. Located 2 km underground to shield it from cosmic rays, PICO-60 is one of several experiments running at SNOLAB in Sudbury, Ontario. It contains 52 kg of the liquid octafluoropropane, which is maintained in a superheated state. If a WIMP collides with a fluorine nucleus, the energy imparted to the fluid will cause it to boil locally, creating a bubble that can be detected using an array of digital cameras and acoustic detectors. While no dark-matter interactions were spotted, the experimental run allows physicists to put new upper limits on the strength of the spin-dependent interaction between WIMPs and protons at WIMP masses between about 10–100 GeV/c2. This will help to guide future dark-matter searches, and the PICO team is now developing a larger version of the detector that will use 500 kg of superheated fluid. The latest results are described on arXiv.

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