Excellent Strategy for Fingerprint Identification using Gold nanoparticles

Identifying fingerprints on paper is a commonly used method in police forensic work, but unfortunately it is not easy to make those fingerprints visible. Now, scientists at the Hebrew University of Jerusalem have developed a new approach for making such fingerprints more readily readable.

The new method, created by a team headed by Prof. Yossi Almog and Prof. Daniel Mandler of the Institute of Chemistry at the Hebrew University, uses an innovative chemical process to produce a negative of the fingerprint image rather than the positive image produced under current methods. Unlike the latter, the Hebrew University-developed process is nearly independent of the composition of the sweat residue left behind on the paper.

In many criminal investigations, paper evidence plays an important role, and it is useful to know who has handled such documents as checks, paper currency, notes, etc. Studies have shown that less than half of the fingerprints on paper items can be made sufficiently visible to enable their identification. The main reason for this seems to be the highly variable composition of the sweat left behind on the paper.

The new procedure developed at the Hebrew University avoids these problems. It involves an inversion of an established method in which gold nanoparticles are first deposited onto the invisible fingerprints, followed by elemental silver, similar to the development of a black and white photograph.

 In the conventional technique, the gold particles get stuck to the amino acid components of the sweat in the fingerprints, and then silver is deposited onto the gold. The result is quite often low-contrast impressions of the fingerprints. In the new method, the gold nanoparticles stick directly to the paper surface, but not the sweat. This technique utilizes the sebum from the fingerprints as a medium to avoid this interference. (Sebum is an oily substance secreted by the sebaceous glands that helps prevent hair and skin from drying out.) Treatment with a developer containing silver then turns the areas with gold on them black, resulting in a clear, negative image of the fingerprint.

Since the method relies only on the fatty components in the fingerprints, the sweaty aspects play no role in the imaging process This technique also promises to alleviate another problem; for example if paper has become wet, it has previously been difficult to detect fingerprints because the amino acids in the sweat, which are the primary substrate for current chemical enhancement reactions, are dissolved and washed away by water, whereas the fatty components are barely affected. Thus, the avoidance of the sweat aspect provides a further enhancement for police laboratory.

Emerging idea of cooling of nanoscale Computer chips by Crystals

Researchers at the Carnegie Institution have discovered a new efficient way to pump heat using crystals. The crystals can pump or extract heat, even on the nanoscale, so they could be used on computer chips to prevent overheating or even meltdown, which is currently a major limit to higher computer speeds.

Researchers at the University of Chicago performed the preliminary simulations on ferroelectric crystals materials that have electrical polarization in the absence of an electric field. The electrical polarization can be reversed by applying an external electrical field. The scientists found that the introduction of an electric field causes a giant temperature change in the material, dubbed the electrocaloric effect (a phenomenon in which a material shows a reversible temperature change under an applied electric field), far above a temperature to a so-called paraelectric state.

The electrocaloric effect pumps heat through changing temperature by way of an applied electric field. The effect has been known since the 1930s, but has not been exploited because people were using materials with high transition temperatures. So low transition temperature materials are preferred, as in that way, the effect is larger if the ambient temperature is well above the transition temperature,

Ferroelectrics become paraelectric, that is, have no polarization under zero electric field above their transition temperature, which is the temperature at which a material changes its state from ferroelectric to paraelectric.

 
Scientists used atomic-scale molecular dynamics simulations, where they followed the behavior of atoms in the ferroelectric lithium niobate as functions of temperature and an electrical field.

Vortex Beams opens new possibilities for electron microscopy

Vortex beams render completely new possibilities for electron microscopy. A method of producing extremely intense vortex beams has been discovered at the Vienna University of Technology (TU Vienna).

Nowadays, electron microscopes are an essential tool, especially in the field of materials science. At TU Vienna, electron beams are being created that possess an inner rotation; these vortex beams cannot only be used to display objects, but to investigate material-specific properties with minute precision. A new breakthrough in research now allows scientists to produce much more intense vortex beams than ever before.

In a tornado, the individual air particles do not necessarily rotate on their own axis, but the air suction overall creates a powerful rotation. The rotating electron beams that have been generated at TU Vienna behave in a very similar manner. Vortex beams can only be explained in terms of quantum physics: the electrons behave like a wave, and this quantum wave can rotate like a tornado or a water current behind a ship's propeller.

After the vortex beam gains angular momentum, it can also transfer this angular momentum to the object that it collides. The angular momentum of the electrons in a solid object is closely linked to its magnetic properties. For materials science it is therefore a huge advantage to be able to make statements regarding angular momentum conditions based on these new electron beams.

Peter Schattschneider and Michael Stöger-Pollach (USTEM, TU Vienna) have been working together with a research group from Antwerp on creating the most intense, clean and controllable vortex beams possible in a transmission electron microscope. The first successes were achieved two years ago: at the time, the electron beam was shot through a minuscule grid mask, whereby it split into three partial beams: one turning right, one turning left and one beam that did not rotate.

Now, a new, much more powerful method has been developed: researchers use a screen, half of which is covered by a layer of silicon nitride. This layer is so thin that the electrons can penetrate it with hardly any absorption, however they can be suitably phase-shifted. After focusing using a specially adapted astigmatic lens, an individual vortex beam is obtained.

 

More exotic applications of vortex beams are also conceivable: in principle, it is possible to set all kinds of objects in rotation, even individual molecules using these beams, which possess angular momentum. Vortex beams could therefore also open new doors in nanotechnology.

Phosphor removal from nano iron

A professor at Michigan State University is part of a team developing a new method of removing phosphorus from wastewater; a problem seriously affecting lakes and streams across the world.

Phosphorus is part of all food as well as is in items such as detergents and fertilizer and remains a critical problem as it is always present in human and animal wastes.

Discharge from human and industrial wastewater and runoff into lakes and streams can cause eutrophication, making the water unsuitable for recreational purposes and reducing fish populations, as well as causing the growth of toxic algae.

Researchers have figured out and tested over the past 10 years is how to produce a media, enhanced with nanoparticles composed of iron, that can more efficiently remove larger amounts of phosphorus from water.

Phosphorus that is dissolved in wastewater, like sugar in water, is hard to remove. A nano-media made with waste iron can efficiently absorb it, making it a solid that can be easily and efficiently removed and recovered for beneficial reuse. Their method of phosphorus retrieval is much more cost effective than processing phosphate rock. Research suggests that it is significantly cheaper to recover phosphorus this way.

Molybdenum disulfide (MoS2): New nanomaterial with several advantages

The discovery of graphene, a material just one atom thick and possessing exceptional strength and other novel properties, started an avalanche of research around its use for everything from electronics to optics to structural materials. But new research suggests that was just the beginning: A whole family of two-dimensional materials may open up even broader possibilities for applications that could change many aspects of modern life.

The latest new material, molybdenum disulfide (MoS₂) was first described just a year ago by researchers in Switzerland. But in that year, researchers at MIT who struggled for several years to build electronic circuits out of graphene with very limited results (have already succeeded in making a variety of electronic components from MoS₂. They say the material could help usher in radically new products, from whole walls that glow to clothing with embedded electronics to glasses with built-in display screens.

 Researchers think graphene and MoS₂ are just the beginning of a new realm of research on two-dimensional materials. Like graphene, itself a 2-D form of graphite, molybdenum disulfide has been used for many years as an industrial lubricant. But it had never been seen as a 2-D platform for electronic devices until last year, when scientists at the Swiss university produced a transistor on the material.

 Then MIT researchers found a good way to make large sheets of the material using a chemical vapor deposition process. As there are lots of hindrance in making electronic products out of graphene due to lack of bandgap, MoS₂ just naturally comes with large band gap.

 MoS₂ is widely produced as a lubricant and as others are working on making it into large sheets, scaling up production of the material for practical uses should be much easier than with other new materials. People are able to fabricate a variety of basic electronic devices on the material: an inverter, which switches an input voltage to its opposite; a NAND gate, a basic logic element that can be combined to carry out almost any kind of logic operation; a memory device, one of the key components of all computational devices; and a more complex circuit called a ring oscillator, made up of 12 interconnected transistors, which can produce a precisely tuned wave output.

One potential application of the new material is large-screen displays such as television sets and computer monitors, where a separate transistor controls each pixel of the display. Because the material is just one molecule thick, unlike the highly purified silicon that is used for conventional transistors and must be millions of atoms thick, even a very large display would use only an infinitesimal quantity of the raw materials. This could potentially reduce cost and weight and improve energy efficiency.

Further reading:

arxiv.org/pdf/1208.1078 

 

In the future, it could also enable entirely new kinds of devices. The material could be used, in combination with other 2-D materials, to make light-emitting devices. Instead of producing a point source of light from one bulb, an entire wall could be made to glow, producing softer, less glaring light. Similarly, the antenna and other circuitry of a cellphone might be woven into fabric, providing a much more sensitive antenna that needs less power and could be incorporated into clothing.

 The material is so thin that it's completely transparent, and it can be deposited on virtually any other material. For example, MoS₂ could be applied to glass, producing displays built into a pair of eyeglasses or the window of a house or office.

100,000 Dots-Per-Inch (DPI) image resolution is achieved using metal-laced nanostructures

Researchers from Institute of Materials Research and Engineering (IMRE) have developed an innovative method for creating sharp, full-spectrum colour images at 100,000 dots per inch (dpi), using metal-pattern nanostructures, without the need for inks. In comparison, current industrial printers such as inkjet and laser jet printers can only achieve up to 10,000 dpi while research grade methods are able to dispense dyes for only single colour images. This novel breakthrough using lithographic technique which can potentially revolutionise the way images are printed and be developed for use in high-resolution reflective colour displays as well as high density optical data storage.

 The inspiration for the research was derived from stained glass, which is traditionally made by mixing tiny fragments of metal into the glass. It was found that nanoparticles from these metal fragments scattered light passing through the glass to give stained glass its colours. Using a similar concept with the help of modern nanotechnology tools, the researchers precisely patterned metal nanostructures, and designed the surface to reflect the light to achieve the colour images.

The resolution of printed colour images very much depends on the size and spacing between individual nanodots. The closer the dots are together and because of their small size, the higher the resolution of the image. With the ability to accurately position these extremely small colour dots, the highest theoretical print colour resolution of 100,000 dpi could be achieved.

Instead of using different dyes for different colours, colour information is encoded into the size and position of tiny metal disks. These disks then interacted with light through the phenomenon of plasmon resonances. Nanostructure pattern, size and spacing are then correlated with the database of colour. These nanostructures were then positioned accordingly.

Mechanical Device invented to measure the mass of a single molecule

A team led by scientists at the California Institute of Technology (Caltech) have made the first-ever mechanical device that can measure the mass of individual molecules one at a time.

This new technology, the researchers say, will eventually help doctors diagnose diseases, enable biologists to study viruses and probe the molecular machinery of cells, and even allow scientists to better measure nanoparticles and air pollution.

The device, which is only a couple millionths of a meter in size, consists of a tiny, vibrating bridge-like structure. When a particle or molecule lands on the bridge, its mass changes the oscillating frequency in a way that reveals how much the particle weighs.

The new instrument is based on a technique Roukes and his colleagues developed over the last 12 years. In work published in 2009, they showed that a bridge-like nanoelectromechanical device could indeed measure the masses of individual particles, which were sprayed onto the apparatus. The difficulty, however, was that the measured shifts in frequencies depended not only on the particle's actual mass, but also on where the particle landed. Without knowing the particle's landing site, the researchers had to analyze measurements of about 500 identical particles in order to pinpoint its mass.

But with the new and improved technique, the scientists need only one particle to make a measurement. To do so, the researchers analyzed how a particle shifts the bridge's vibrating frequency. All oscillatory motion is composed of so-called vibrational modes. If the bridge just shook in the first mode, it would sway side to side, with the center of the structure moving the most. The second vibrational mode is at a higher frequency, in which half of the bridge moves sideways in one direction as the other half goes in the opposite direction, forming an oscillating S-shaped wave that spans the length of the bridge. There is a third mode, a fourth mode, and so on. Whenever the bridge oscillates, its motion can be described as a mixture of these vibrational modes.

The team found that by looking at how the first two modes change frequencies when a particle lands, they could determine the particle's mass and position. Traditionally, molecules are weighed using a method called mass spectroscopy, in which tens of millions of molecules are ionized -- so that they attain an electrical charge -- and then interact with an electromagnetic field. By analyzing this interaction, scientists can deduce the mass of the molecules.

The problem with this method is that it does not work well for more massive particles which have a harder time gaining an electrical charge. As a result, their interactions with electromagnetic fields are too weak for the instrument to make sufficiently accurate measurements.

The new device, on the other hand, does work well for large particles. In fact, the researchers say, it can be integrated with existing commercial instruments to expand their capabilities, allowing them to measure a wider range of masses.

The researchers demonstrated how their new tool works by weighing a molecule called immunoglobulin, an antibody produced by immune cells in the blood. By weighing each molecule, which can take on different structures with different masses in the body, the researchers were able to count and identify the various types of immunoglobulin. Not only was this the first time a biological molecule was weighed using a nanomechanical device, but the demonstration also served as a direct step toward biomedical applications. Future instruments could be used to monitor a patient's immune system or even diagnose immunological diseases.

In the more distant future, the new instrument could give biologists a view into the molecular machinery of a cell. Proteins drive nearly all of a cell's functions, and their specific tasks depend on what sort of molecular structures attach to them -- thereby adding more heft to the protein -- during a process called posttranslational modification. By weighing each protein in a cell at various times, biologists would now be able to get a detailed snapshot of what each protein is doing at that particular moment in time.

Another advantage of the new device is that it is made using standard, semiconductor fabrication techniques, making it easy to mass-produce. That's crucial, since instruments that are efficient enough for doctors or biologists to use will need arrays of hundreds to tens of thousands of these bridges working in parallel. 

Newest Material for Optical Applications

Researchers have been successful in developing a structure that could bring optical advances including ultrapowerful microscopes, computers and solar cells. They have shown how to create the metamaterials without the traditional silver or gold previously required. Using the metals is impractical for industry because of high cost and incompatibility with semiconductor manufacturing processes. The metals also do not transmit light efficiently, causing much of it to be lost. The Purdue researchers replaced the metals with an aluminum-doped zinc oxide (AZO).

This new metamaterial consists of 16 layers alternating between AZO and zinc oxide. Light passing from the zinc oxide to the AZO layers encounters an extreme anisotropy, causing its dispersion to become hyperbolic, which dramatically changes the light's behaviour. The doped oxide brings not only enhanced performance but also is compatible with semiconductors. Metamaterials can be applied in optical microscopes that would make them 10 times more powerful and able to see objects as small as DNA; and also useful in advanced sensors; more efficient solar collectors; quantum computing; and cloaking devices. The AZO also modulate the optical properties of metamaterials by varying the concentration of aluminium in the AZO and also by applying an electric filed to the fabricated metamaterial. This switching ability might usher in a new class of metamaterials that could be turned hyperbolic and non-hyperbolic at the flip of a switch.

This could actually lead to a whole new family of devices that can be tuned or switched. AZO can go from dielectric to metallic. So at one specific wavelength, at one applied voltage, it can be metal and at another voltage it can be dielectric. This would lead to tremendous changes in functionality.

The researcher doped zinc oxide with aluminum, meaning the zinc oxide is impregnated with aluminum atoms to alter the material's optical properties. Doping the zinc oxide causes it to behave like a metal at certain wavelengths and like a dielectric at other wavelengths.

The material has been shown to work in the near-infrared range of the spectrum, which is essential for optical communications, and could allow researchers to harness optical black holes to create a new generation of light-harvesting devices for solar energy applications.

 Current optical technologies are limited because, for the efficient control of light, components cannot be smaller than the size of the wavelengths of light. Metamaterials are able to guide and control light on all scales, including the scale of nanometers, or billionths of a meter.

Unlike natural materials, metamaterials are able to reduce the index of refraction to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. It causes the bent-stick-in-water effect, which occurs when a stick placed in a glass of water appears bent when viewed from the outside. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material

Natural materials typically have refractive indices greater than one. Metamaterials, however, can make the index of refraction vary from zero to one, which possibly will enable applications including the hyperlens.

Alternative plasmonic materials such as AZO overcome the bottleneck created by conventional metals in the design of optical metamaterials and enable more efficient devices.

Light replaces electricity through metatronics

The technological world of the 21^st century owes a tremendous amount to advances in electrical engineering, specifically, the ability to finely control the flow of electrical charges using increasingly small and complicated circuits. And while those electrical advances continue to race ahead, researchers at the University of Pennsylvania are pushing circuitry forward in a different way, by replacing electricity with light.

Different arrangements and combinations of electronic circuits have different functions, ranging from simple light switches to complex supercomputers. These circuits are built of different arrangements of circuit elements, for example resistors, inductors and capacitors, which manipulate the flow of electrons in a circuit in mathematically precise ways.

Now, researchers at Penn have created the first physical demonstration of lumped optical circuit elements. This represents a milestone in a nascent field of science and engineering. In electronics, the lumped designation refers to elements that can be treated as a black box, something that turns a given input to a perfectly predictable output without an engineer having to worry about what exactly is going on inside the element. Optics has always had its own set of elements, things like lenses, waveguides and gratings, but they were never lumped. Those elements are all much larger than the wavelength of light because that's all that could be easily built in the old days. For electronics, the lumped circuit elements were always much smaller than the wavelength of operation, which is in the radio or microwave frequency range.

 Nanotechnology has now opened that possibility for lumped optical circuit elements, allowing construction of structures that have dimensions measured in nanometers. In this experiment's case, the structure was comb-like arrays of rectangular nanorods made of silicon nitrite.

 The "meta" in "metatronics" refers to metamaterials, the relatively new field of research where nanoscale patterns and structures embedded in materials allow them to manipulate waves in ways that were previously impossible. Here, the cross-sections of the nanorods and the gaps between them form a pattern that replicates the function of resistors, inductors and capacitors, three of the most basic circuit elements, but in optical wavelengths.

 In their experiment, the researchers illuminated the nanorods with an optical signal, a wave of light in the mid-infrared range. They then used spectroscopy to measure the wave as it passed through the comb. Repeating the experiment using nanorods with nine different combinations of widths and heights, the researchers showed that the optical current and optical voltage were altered by the optical resistors, inductors and capacitors with parameters corresponding to those differences in size.

A section of the nanorod acts as both an inductor and resistor, and the air gap acts as a capacitor.

 Beyond changing the dimensions and the material the nanorods are made of, the function of these optical circuits can be altered by changing the orientation of the light, giving metatronic circuits access to configurations that would be impossible in traditional electronics. This is because a light wave has polarizations; the electric field that oscillates in the wave has a definable orientation in space. In metatronics, it is that electric field that interacts and is changed by elements, so changing the field's orientation can be like rewiring an electric circuit.

 When the plane of the field is in line with the nanorods, the circuit is wired in parallel and the current passes through the elements simultaneously. When the plane of the electric field crosses both the nanorods and the gaps, the circuit is wired in series and the current passes through the elements sequentially.

This principle could be taken to an even higher level of complexity by building nanorod arrays in three dimensions. An optical signal hitting such a structure's top would encounter a different circuit than a signal hitting its side. Another reason for success in electronics has to do with its modularity.

Smallest transistor ever built: New Beginning of Quantum Computing

The smallest transistor has been created using a single phosphorus atom by an international team of researchers at the University of New South Wales, Purdue University and the University of Melbourne.

Michelle Simmons, group leader and director of the ARC Centre for Quantum Computation and Communication at the University of New South Wales, says the development is less about improving current technology than building future tech.

This is a beautiful demonstration of controlling matter at the atomic scale to make a real device, Simmons says. "Fifty years ago when the first transistor was developed, no one could have predicted the role that computers would play in our society today. As we transition to atomic-scale devices, we are now entering a new paradigm where quantum mechanics promises a similar technological disruption. It is the promise of this future technology that makes this present development so exciting.

Gerhard Klimeck, who directed the Purdue group that ran the simulations, says this is an important development because it shows how small electronic components can be engineered.

Moore’s Law simply stated that the number of transistors that can be placed on a processor will double approximately every 18 months. The latest Intel chip, the "Sandy Bridge," uses a manufacturing process to place 2.3 billion transistors 32 nanometers apart. A single phosphorus atom, by comparison, is just 0.1 nanometers across, which would significantly reduce the size of processors made using this technique, although it may be many years before single-atom processors actually are manufactured. The single-atom transistor does have one serious limitation: It must be kept very cold, at least as cold as liquid nitrogen, or minus 391 degrees Fahrenheit (minus 196 Celsius).

The atom sits in a well or channel, and for it to operate as a transistor the electrons must stay in that channel. At higher temperatures, the electrons move more and go outside of the channel. For this atom to act like a metal you have to contain the electrons to the channel. If someone develops a technique to contain the electrons, this technique could be used to build a computer that would work at room temperature. But this is a fundamental question for this technology.

Although single atoms serving as transistors have been observed before, this is the first time a single-atom transistor has been controllably engineered with atomic precision. The structure even has markers that allow researchers to attach contacts and apply a voltage. The thing that is unique about what is done here, with atomic precision, individual atom is positioned within the device.

Simmons says this control is the key step in making a single-atom device. By achieving the placement of a single atom, we have, at the same time, developed a technique that will allow us to be able to place several of these single-atom devices towards the goal of a developing a scalable system.

The single-atom transistor could lead the way to building a quantum computer that works by controlling the electrons and thereby the quantum information, or quantum bits. Some scientists, however, have doubts that such a device can ever be built.

 Whilst this result is a major milestone in scalable silicon quantum computing, it does not answer the question of whether quantum computing is possible or not," Simmons says. The answer to this lies in whether quantum coherence can be controlled over large numbers of quantum bits. The technique developed is potentially scalable, using the same materials as the silicon industry, but more time is needed to realize this goal.

Nanoscale wires defy quantum predictions

http://www.nature.com/news/nanoscale-wires-defy-quantum-predictions-1.9747

Miracle Antennas for Optical Innovations

Researchers have shown how arrays of tiny plasmonic nanoantennas are able to precisely manipulate light in new ways that could make possible a range of optical innovations such as more powerful microscopes, telecommunications and computers.

The researchers at Purdue University used the nanoantennas to abruptly change the phase of light phase. Light is transmitted as waves analogous to waves of water, which have high and low points. The phase defines these high and low points of light.

By abruptly changing the phase light propagates opens up the possibility of many potential applications. , Harvard researchers modified Snell's law, a long-held formula used to describe how light reflects and refracts, or bends, while passing from one material into another.

Until now, Snell's law has implied that when light passes from one material to another there are no abrupt phase changes along the interface between the materials. Harvard researchers, however, conducted experiments showing that the phase of light and the propagation direction can be changed dramatically by using new types of structures called metamaterials, which in this case were based on an array of antennas.

The wavelength size manipulated by the antennas in the Purdue experiment ranges from 1 to 1.9 microns.

The near infrared, specifically a wavelength of 1.5 microns, is essential for telecommunications. Information is transmitted across optical fibers using this wavelength, which makes this innovation potentially practical for advances in telecommunications.

The Harvard researchers predicted how to modify Snell's law and demonstrated the principle at one wavelength.

The innovation could bring technologies for steering and shaping laser beams for military and communications applications, nanocircuits for computers that use light to process information, and new types of powerful lenses for microscopes.

Critical to the advance is the ability to alter light so that it exhibits anomalous behaviour: notably, it bends in ways not possible using conventional materials by radically altering its refraction, a process that occurs as electromagnetic waves, including light, bend when passing from one material into another.

Scientists measure this bending of radiation by its index of refraction. Refraction causes the bent-stick-in-water effect, which occurs when a stick placed in a glass of water appears bent when viewed from the outside. Each material has its own refraction index, which describes how much light will bend in that particular material. All natural materials, such as glass, air and water, have positive refractive indices. However, the nanoantenna arrays can cause light to bend in a wide range of angles including negative angles of refraction.

Importantly, such dramatic deviation from the conventional Snell's law governing reflection and refraction occurs when light passes through structures that are actually much thinner than the width of the light's wavelengths, which is not possible using natural materials. Also, not only the bending effect, refraction, but also the reflection of light can be dramatically modified by the antenna arrays on the interface, as the experiments showed.

The nanoantennas are V-shaped structures made of gold and formed on top of a silicon layer. They are an example of metamaterials, which typically include so-called plasmonic structures that conduct clouds of electrons called plasmons. The antennas themselves have a width of 40 nanometers, or billionths of a meter, and researchers have demonstrated they are able to transmit light through an ultrathin plasmonic nanoantenna layer about 50 times smaller than the wavelength of light it is transmitting.

 This ultrathin layer of plasmonic nanoantennas makes the phase of light change strongly and abruptly, causing light to change its propagation direction, as required by the momentum conservation for light passing through the interface between materials.