Monthly Archives: August 2013

No-longer-missing links for 29/08/13

Ugly Animal Preservation Society seeks new mascot

Working in partnership with the National Science + Engineering Competition, the Ugly Animal Preservation Society are running a public vote in order to find a new mascot  – no cuddly pandas here!

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Megan Scudellari wins Clark/Payne Award

The Clark/Payne Award encourages young science writers by recognizing outstanding reporting in all fields of science. It is given each year in memory of journalists Ev Clark and Seth Payne, who offered friendship and advice to a generation of young reporters.

Does this photograph show Mary Anning?

Renowned geologist and fossil hunter Marry Anning still remains elusive to modern audiences. 

Maths as magic

Chalk, equations and mathematicians come alive in Brian Sanderson’s young adult novel The Rithmatist.

Scientists as public communicators

How do scientists view the media, and how do they contribute towards public awareness of science?

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The Natural Abundance of Elements

We’ve talked a lot about the different chemical elements and the periodic table, so I loved finding the infographic below which shows the natural abundance of those elements in the universe, inside the Earth, in the Earth’s crust, in the oceans, in the atmosphere, and in us!

Abundance of Elements

It’s worth clicking through to see more detail, like how the heavier elements are more common in us and the Earth than the oceans or air, and how the noble gases on the right of the table are most common in our atmosphere and in space, whereas we and the Earth and the oceans have more of the reactive elements that ionize easily, those in the farthest left and nearly farthest right columns. Also, see how in the periodic table for the universe, the square for hydrogen is nearly black? Hydrogen is by far the most common element in the universe, and making heavier elements requires nuclear fusion in the heart of a star. It’s cool to see visual reminders of that basic fact, that the elements we rely on have all been built by stars.

Light, Scattering, and Why the Sky is Blue

I’ve been writing more about light recently, so I wanted to cover a basic question that most people first ask as children: why is the sky blue? We can tell that the blue color of the sky is related to sunlight, because  at night, we can see out to the black of space and the stars. We also know it’s related to the atmosphere, because in photos from places like the Moon which have no atmosphere, the sky is black even when the Sun is up. So what’s going on?

When light from the Sun reaches Earth, its photons have a combination of wavelengths (or energies), and we call the sum of all of those the solar spectrum. Some of these wavelengths of light are absorbed by particles in the atmosphere, but others are scattered, which means that the photons in question are deflected to a new direction. Scattering of light happens all around us, because the electromagnetic wave nature of photons makes them very sensitive to variations in the medium through which they travel. Other than absorption of light, scattering is the main phenomenon that affects color.

There are a few different types of scattering. We talked about one type recently, when discussing metamaterials and structural color: light can be scattered by objects that are a size similar to the wavelength of that light. That is called Mie scattering, and it’s why clouds appear solid even though they are mostly empty. The clouds are formed of tiny droplets, around the size of the visible wavelengths of light, and when these droplets scatter white light, the clouds themselves appear diffuse and white. Milk also appears white because it has proteins and fat in tiny droplets, suspended in water, which scatter white light.

However, even objects much smaller than the wavelength of light can induce scattering. The oxygen and nitrogen molecules in the atmosphere can also act as scatterers, in what’s called Rayleigh scattering (or sometimes the Tyndall effect). For Rayleigh scattering, these molecules can be affected by the electromagnetic field that the photon carries. A molecule can be polarized, meaning the positive and negative charges in the molecule move in opposite directions, and then the polarized molecule interacts with the light by scattering it. But, the polarizability of individual molecules depends on the wavelength of the incoming light, meaning that some wavelengths will scatter more strongly than others. When Rayleigh worked out the mathematical form of this dependence, in 1871, he found that the scattering was inversely proportional to the fourth power of the wavelength of light, which means that blue light (which has a smaller wavelength) will scatter much more strongly than red light (which has a larger wavelength).

Thus, we see the Sun as somewhat yellow, because only the longer wavelength light in red and yellow travels directly to us. The shorter wavelength blue light is scattered away into the sky, and comes to our eyes on a very circuitous and scattered route that makes it look like the blue light is coming from the sky itself. At sunset, the sun appears even redder because of the increased amount of atmosphere that the light has travelled through, scattering away even more blue light. And, when there is pollution in the air, the sun can appear redder because there are more scattering centers that scatter away the blue light.

Of course, the fact that blue light scatters more is only half the story. If that were all there is to it, we’d see the sky as a deep violet, because that’s the shortest wavelength of light that our eyes can see. But even though we can see the violet in a rainbow, our eyes are actually much less sensitive to it than they are to blue light. Our eyes perceive color using special neurons called cones, and of the three types of cones, only one can detect blue and violet light. But the blue cone’s response to light peaks at around 450 nm, which is right in the middle of the blue part of the spectrum. So we see the sky as blue because it is the shortest wavelength that we’re capable of detecting in bulk. Different particles in the air can change the color of the sky, but so would different ways of sensing color. So Rayleigh scattering determines which light is scattered, and our visual system determines which of that light we see best: sky blue.

No-longer-missing links for 20/08/13

UK faces desperate shortage of science and maths teachers 

“We’ve now had successive years when public sector wages have been held down, and regular stories about the problems facing the profession. No wonder graduates in the Stem (science, technology, engineering and maths) subjects are accepting jobs elsewhere. That was always the risk.”

Brainiac Live – science communication abuse 

“The biggest irony of Brainiac Live being booked or promoted by STEM engagement organisations is  that it is self-evidently written and performed by people who refuse to believe that science is interesting. This is a capital crime in science communication.”

Scottish culture minister: we need more women in games development 

Although the image of gaming is improving, thanks partly to the ubiquity of smartphones, which have made games more widely available, female developers attending the Protoplay event have come up against familiar barriers. “I always got my brother’s hand-me-down computers – it was seen as a boy’s thing when I was growing up,” says Sophia George, who is about to take up a new position as the Victoria and Albert Museum’s game developer in residence. “I said no, I want to do it. We can’t give up!”

Statistics Every Writer Should Know 

Here, described in plain English, are some basic concepts in statistics that every writer should know…

Pitching Your Passion in 2-3 minutes (an infographic) 

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Plasmons, Shiny Metals, and Stained Glass

Remember plasmas, the phase of matter where atoms are ripped apart into electrons and nuclei? Plasmas are primed for strong electromagnetic interactions with the world around them, because of all the loose charged particles. They can be used to etch down into surfaces and catalyze chemical reactions, though the ions in a plasma won’t necessarily react with every form of matter they come across. And you can actually use an electromagnetic field on its own to contain a plasma, because of the plasma’s sensitivity to electromagnetic force. The most common design for a fusion reactor, the tokamak, uses a doughnut-shaped field to contain a plasma.

That’s how plasmas work at the macroscale, but it’s the individual charged ions in the plasma which react to electromagnetic force. Their interactions sum to a larger observable phenomena, which emerges from nanoscale interactions. But interestingly, the collective interactions of these ions can actually be approximated as discrete entities, called quasiparticles. We’ve talked about quasiparticles before, when we talked about holes which are quasiparticles defined by the absence of an electron. But in plasmas, the collective motion of the ions can also be considered as a quasiparticle, called a plasmon. Each individual ion is responding to its local electromagnetic field, but the plasmon is what we see at a larger scale when everything appears to be responding in unison. A plasmon isn’t actually a particle, just a convenient way to think about collective phenomena.

Plasmons can be excited by an external electromagnetic stimulus, such as light. And actually, anyone who has looked up at a stained glass window has witnessed plasmonic absorption of light! Adding small amounts of an impurity like gold to glass results in a mixture of phases, with tiny gold nanoparticles effectively suspended in the amorphous silica that makes up glass. Gold, like many metals, has a high electron density, and the electrons effectively comprise a plasma within each nanoparticle. When light shines through the colored glass, some wavelengths are plasmonically absorbed and others pass through. Adding a different metal to the glass can change the color, and so can different preparations of the glass that modify the size of the included nanoparticles. So all the colors in the window shown below are due to differing nanoparticles that plasmonically absorb light as it passes through!

Now you might ask, what determines which wavelengths of light pass through and which don’t? In the case of stained glass, it has to do with the size of the nanoparticles and the metal. But more generally, plasmas have a characteristic frequency at which they oscillate most easily, called the plasma frequency. The plasma frequency depends on several fundamental physical constants, including the mass and charge of an electron, but notably it also depends on the density of electrons in the plasma. For nanoparticles, the size of the particle also affects the response frequency. The practical upshot of the plasma frequency, though, is that if incident light has a frequency higher than the plasma frequency, the electrons in the plasma can’t respond fast enough to couple to the light, and it passes through the material. So the material properties that dictate the plasma frequency also determine whether light will be absorbed or transmitted.

For most metals that aren’t nanoscale, the plasma frequency is somewhere in the ultraviolet range of the electromagnetic spectrum.  Thus, incident visible light is reflected by the free electron plasma in the metal, right at the surface of the material. And that’s why metals appear shiny!