Colour patterns – Shifting, shining

Who could have failed to notice the shifting, shining patterns of light playing on the sand below shallow waters or at the bottom of a swimming pool? There is perhaps nothing more relaxing than watching the way the colours weave in and out of each other as waves ripple on the surface. Now, a British physicist believes he knows what causes such colourful displays and the answer lies in the shadowy world of dark light.

When two waves meet their peaks and troughs interfere. If the peaks are in phase one sees a higher net peak as the energies of each wave add together. If two troughs coincide, they subtract from each other making a deeper trough. On the other hand, when a peak and a trough meet they apparently cancel out. For waves on the sea, the effect is a patch of dead water. For light, the picture is altogether more complicated and beautiful as Michael Berry of Bristol University has discovered.

Michael Berry

Michael Berry

Interference of white light produces coloured patterns, explains Berry, because the different wavelengths in the light add and subtract differently at different places. He has combined colour theory and wave physics to illuminate for the first time that calm water – the dark light. He has used a computer model to simulate the interference patterns produced by two light waves. Where peak and trough meet to cancel each other out he sees a region of dark light called a phase singularity.

In these special places the phase of the wave is undefined, just as time is undefined at the North Pole, says Berry. However, the colours hidden in the darkness can be predicted, he adds, By magnifying the intensity there, the colours form characteristic and striking patterns.

Dark light!

Dark light!

These theoretically predicted colours of dark light have yet to be investigated experimentally but Berry’s work predicts many different colour patterns in the dark light.

Random phases, which won first prize in the ’Visions of Science Photographic Awards 2002’ for the category of ’Science as Art’. Please see the website at: http://www.visions-of-science.co.uk/

Random phases, which won first prize in the ’Visions of Science Photographic Awards 2002’ for the category of ’Science as Art’. Please see the website at: http://www.visions-of-science.co.uk/

For instance, random light waves, interfere to make delicate features, like the shimmering light at the bottom of a swimming pool, where the waves appear to move faster than light. One might say that Berry has unwoven the rainbow a little further than Sir Isaac Newton ever did with his prisms and shafts of light in a darkened room. Berry is showing that phase singularities lie at the heart of a most beautiful and relaxing natural phenomenon.

Berry published more details of his calculations in the New Journal of Physics.

Further reading

New J. Phys., 4, 74 (2002)
http://www.iop.org/EJ/S/UNREG/i2Q01uCcGWO1IGWmOlARJw/article/1367-2630/4/1/374/nj2174.html

Michael Berry
http://www.phy.bris.ac.uk/people/berry_mv/

Suggested searches

Interference light waves
Colour

What on Earth?

Four billion years ago, when the Earth was but a mere babe nestled in the cradle of the solar accretion disc, the local environs were littered with asteroid-sized chunks of rock known as planetesimals. The Period of Heavy Bombardment, which lasted from about 4.5 to 3.8 billion years ago, saw the planetesimals colliding with the Earth on a daily basis. Not the gentlest of starts to life.

In October, delegates at Perspectives in Astrobiology, a NATO Advanced Studies Institute in Chania, Crete, heard how there is something anomalous about the end of the Period of Heavy Bombardment. We know from the fossil record that life has existed on Earth since around 3.8 billion years ago but how could it have survived and indeed thrived during the daily onslaught of the PHB?

Could the Moon harbour the secrets of life on Earth? (Credit: NASA)

Could the Moon harbour the secrets of life on Earth? (Credit: NASA)

Some astrobiologists believe those bombarding chunks of rock, or perhaps a comet, may have carried the seeds of life to Earth, explaining its origins during the PHB. Others wonder whether the rocky colliders simply deposited the raw materials, organic building blocks, ready for Earth’s seemingly special conditions to trigger the emergence of life. There is no clear answer yet.

Astrobiologists, however, think they now have a source of rock from that epoch that is truly out of this world and could allow them to study the molecular fossils from the dawn of time as well as revealing much about the wind and rain, earthquakes and plate tectonics on Earth at that time.

John Armstrong

John Armstrong

But, where is this ancient record of Earth’s early days?

Guillermo Gonzalez

Guillermo Gonzalez

According to John Armstrong and Llyd Wells of the University of Washington and Guillermo Gonzalez at Iowa State University it is on the Moon.

A large Moon rock nicknamed Big Muley, weighing 11.7 kg. This rock was the largest returned to Earth by Apollo astronauts. (Credit: NASA)

A large Moon rock nicknamed Big Muley, weighing 11.7 kg. This rock was the largest returned to Earth by Apollo astronauts. (Credit: NASA)

All those repeated collisions had quite an impact on the Earth, flinging off great pieces of debris into space. 4.5 billion years ago, astronomers believe, the Moon itself was simply another piece of space debris, the original chip off the old block, trapped in Earth’s orbit after a particularly violent collision between Earth and a Mars-sized planetesimal. During the PHB plenty more debris was hewn from the Earth and would have been cleaned up by the Moon when it was much closer to the Earth than it is today, as it orbited the young Earth.

The lack of weather and plate tectonics on the Moon mean, much of that debris might still be lying still on the Moon’s surface. The researchers suggest that at least 20 tonnes of Earth material might cover every 100 square kilometre of the Moon’s surface. The Apollo missions brought back to Earth a mere fraction of the Moon’s surface rocks and soils, some 400 kg. Could there be some non-Lunar material in those samples? Armstrong reckons it is a possibility and that samples from the Moon’s eastern limb, as seen from Earth might be the place to look, because that would have likely gathered up the most debris from Earth’s orbit.

Spotting the chemical differences between true Moon rock and more recently acquired Earth debris on the Moon’s surface will be key to determining the original origin of a sample. The presence of hydrated minerals or hydrocarbons would be the first characteristics to investigate.

Further reading

John Armstrong
http://space.weber.edu/~jca/HomePage/John_Armstrong.html

Guillermo Gonzalez
http://www.public.iastate.edu/~astro/faculty/gonzalez.html

Suggested searches

Astrobiology
Lunar surface
Moon formation
Moon

Testing, testing

The chemical industry is faced with an interminable job in putting its products through environmental risk assessment, safety testing and other reviews. Indeed, the British Government demands that chemicals undergo a rigorous testing programme, including an ‘environmental risk assessment’, before it is satisfied a substance is safe. Once a material has full marks it will then be granted a sales licence for the UK and Europe.

However, the regulations date back to 1981 and there is now a massive backlog of tens of thousands of chemicals which require checking. A new spin out company from Newcastle University hopes to help solve the backlog problem in the chemicals industry. Enviresearch is using computer models to determine whether chemicals are environmentally friendly.

James Garratt

James Garratt

The models use known relationships between chemical properties and environmental behaviour, explains Enviresearch company director James Garratt of the School of Biology. They work by bringing together all available information about the chemical properties to predict where the chemical will go and what effect it will have. He adds that risk assessments can be done much more quickly and cheaply on the computer than using time-consuming laboratory tests.

Computer modelling provides results in several days, as opposed to the several weeks needed for laboratory tests to be completed. In many cases, sufficient data have already been produced, and all that is needed is the right analysis, says Garratt. We are currently able to test a wide range of chemicals, including pesticides, paints, detergents and wood preservatives for our clients using a wide range of mathematical models.

PELMO screenshot

PELMO screenshot

The models with bizarre names such as TOXSWA, TOXic substances in Surface Waters, PELMO, Pesticide Leaching MOdel and EUSES, European Union System for the Evaluation of Substances, carry out a detailed analysis of information about a particular chemical and predict what would happen if that substance were to enter the environment through the soil or water body.

The model might, for instance, reveal a substance to be benign or in the worst case scenario to show that it is harmful to birds and fish or otherwise seriously damaging to an ecosystem. The predicted concentrations of the chemical in the environment are compared to the toxicity, adds Garratt. If the first concentration is higher than the second, then alarm bells start to ring.

There are many variables to consider in running a model for risk assessment. The acidity of the soil, for instance, can alter the risk associated with a particular pesticide and whether or not that compound would be likely to contaminate the water supply. If a compound is volatile, then the concentration in soil or water might fall quickly, but pollution of the air will increase.

Enviresearch’s main client groups using these services are in the agrochemical industry and the chemical testing industry. There is increasing interest from the chemical industry, especially from the smaller, speciality chemical companies.

Another issue that is beginning to have an impact on these companies is the EU Chemicals Policy Review. The EU issued a White Paper in 2001 proposing that all chemicals placed on the market are registered and their risks in use evaluated with an authorisation procedure for chemicals of very high concern. The aim of the Review will be a new and comprehensive testing program for chemicals with the aim of protecting human health and the environment. However, according to the UK’s Chemical Industries Association, with trades union backing and support from the Confederation of British Industry, the new attitude to chemicals testing could be very harmful to the British chemical industry and especially to small to medium enterprises (SMEs).

Enviresearch believes that modelling can provide a cost- effective solution to many of the regulatory problems facing SMEs, Garratt says. One aspect in particular that greatly concerns the chemical industry as a whole and the animal rights movement is that the new testing policy will lead to a massive increase in the numbers of animals used in testing programs. Again, modelling can optimise the use of existing data and restrict the number of animal experiments that need to be performed, adds Garratt.

Further reading

Enviresearch
http://www.enviresearch.com/

TOXSWA
http://eco.wiz.uni-kassel.de/model_db/mdb/toxswa.html

PELMO
http://www.wiz.uni-kassel.de/model_db/mdb/pelmo.html

EUSES
http://ecb.jrc.ec.europa.eu/Euses/

Chemical Industries Association
http://www.cia.org.uk/

Confederation of British Industry
http://www.cbi.org.uk/ndbs/staticpages.nsf/StaticPages/home.html/?OpenDocument

Suggested searches

Chemicals safety