Aside

The nature of polarized light using smartphones

malus
Martín Monteiro, Cecilia Stari, Cecilia Cabeza, Arturo C. Marti

Originally an empirical law, nowadays Malus’ is seen as a key experiment to demonstrate the traversal nature of electromagnetic waves, as well as the intrinsic connection between optics and electromagnetism. More specifically, it is an operational way to characterize a linear polarized electromagnetic wave. A simple and inexpensive setup is proposed in this work, to quantitatively verify the nature of polarized light. A flat computer screen serves as a source of linear polarized light and a smartphone is used as a measuring instrument thanks to its built-in sensors. The intensity of light is measured by means of the luminosity sensor with a tiny filter attached over it. The angle between the plane of polarization of the source and the filter is measured by means of the three-axis accelerometer, that works, in this case, as an incliometer. Taken advantage of the simultaneous use of these two sensors, a complete set of measures can be obtained just in a few seconds. The experimental light intensity as a function of the angle shows an excellent agreement with standard results…
…. Read more at https://arxiv.org/ftp/arxiv/papers/1607/1607.02659.pdf

Advertisements
Aside

Vampire Selfie: A Curious Case of an Absent Reflection

Top view of a person standing in an elevator while facing the door. (a) Rays originating from a point (O) on an object, reflected from a plane surface, form a virtual image at the apparent point of origin (I) of the reflected rays. (b) Rays originating from a point on an object, reflected from many irregularly oriented small facets, cannot be traced back to an apparent common point of origin, so no image forms. (c) If many points (O1, O2, O3, etc.) along the object are approximately the same color and shape, then the randomly reflected rays from these various points can appear to have a common origin (I) and form an image.

Top view of a person standing in an elevator while facing the door. (a) Rays originating from a point (O) on an object, reflected from a plane surface, form a virtual image at the apparent point of origin (I) of the reflected rays. (b) Rays originating from a point on an object, reflected from many irregularly oriented small facets, cannot be traced back to an apparent common point of origin, so no image forms. (c) If many points (O1, O2, O3, etc.) along the object are approximately the same color and shape, then the randomly reflected rays from these various points can appear to have a common origin (I) and form an image.

Joshua M. Grossman
During a recent ride in an elevator, I was startled by an observation. Once the door closed, the features on the back wall of the elevator were evident in a reflection on the door; however, my own reflection appeared absent . How could that be? What physics caused this curious phenomenon? The elevator had wooden molding, including horizontal strips that ran all the way around the back and sides . These horizontal strips were what showed up most clearly in the reflection. The door’s surface was brushed metal with the brush marks all running vertically. Therein lay the solution…
…Read more at scitation.aip.org

Light can break Newton’s third law – by cheating

dn24411-1_1072
by Michael Slezak
Isaac Newton just got cheated. Laser pulses have been made to accelerate themselves around loops of optical fibre, seeming to break the physicist’s law that every action must have an equal and opposite reaction. The work exploits a trick with light that only makes it appear to have mass, so it is a bit of a cheat, but it may one day lead to faster electronics and more reliable communications.

According to Newton’s third law of motion, when one billiard ball strikes another, the two balls should bounce away from each other. But if one of the billiard balls had a negative mass, then when the two balls collide they will accelerate in the same direction. This effect could be useful in a diametric drive, a speculative “engine” in which negative and positive mass interact to accelerate forever. NASA explored using the effect in the 1990s in a bid to make a diametric drive for better spacecraft propulsion. But there was a very big fly in the ointment: quantum mechanics states that matter cannot have a negative mass. Even antimatter, made of particles with the opposite charge and spin to their normal matter counterparts, has positive mass.

“Writing a negative mass in quantum field theory doesn’t make any difference,” says Archil Kobakhidze at the University of Sydney, Australia. The equations involve terms that are always squares of mass, so any negative mass will become positive anyway. “It has no observable meaning.”

Mass effect

Now Ulf Peschel at the University of Erlangen-Nuremberg in Germany and his colleagues have made a diametric drive using “effective mass”. As photons travel at the speed of light they have no rest mass. But if you shine pulses of light into some layered materials, such as crystals, some of the photons can be reflected backwards by one layer and then reflected forwards again by another. That delays part of the pulse, causing it to interfere with the rest of the pulse as it propagates more slowly through the material.

“It’s a bit like what happens with a stroboscope,” says Dragomir Neshev at the Australian National University in Canberra, who was not involved in the study. If you watch a spoked wheel turning under a strobe it can appear to move at a different speed or even backwards.

When a material slows the speed of the pulse proportional to its energy, it is behaving as if it has mass – called effective mass. Depending on the shape of the light waves and the structure of the crystal, light pulses can have a negative effective mass. But to get such a pulse to interact with one with a positive effective mass requires a crystal that is so long it would absorb the light before the two pulses could show a diametric drive effect.

To get around this, Peschel created a series of laser pulses in two loops of fibre-optic cable. The pulses get split between the loops at a contact point, and the light keeps moving around each loop in the same direction. The key is that one loop is slightly longer than the other, so light going around the longer loop is relatively delayed (see diagram, above right). When that pulse comes back around and splits at the contact point, it shares some of its photons with pulses in the other loop. After a few round trips, the pulses develop an interference pattern that gives them effective mass.

Clever loops

The team created pulses with positive and negative effective mass. When the opposing pulses interacted in the loops, they accelerated in the same direction, moving past the detectors a little bit sooner on each round trip.

“By having these loops you can loop it forever  – it’s equivalent to having enormously long crystals,” says Neshev, whose group has also tried to create a diametric drive. “It is nice physics and a very clever apparatus.”

Electrons in semiconductors can also have effective mass, so the loops could be used to speed them up and boost processing power in computers, says Peschel. And in some fibres the speed of a light pulse is equivalent to its wavelength, which means the loops could be used to control a fibre’s colour output. Neshev says the method could increase the bandwidth of optical communications or even help create bright displays like laser screens. But he cautions that it will not be easy to adapt the loops for practical purposes.

Journal reference: Nature Physics, DOI: 10.1038/NPHYS2777

Read more at http://www.newscientist.com/article/dn24411#.Ul20k1DIbQw

Optical “Bernoulli” Forces

Light scattering from a rotating dielectric cylinder

Light scattering from a rotating dielectric cylinder

Ramis Movassagh and Steven G. Johnson
By Bernoulli’s law, an increase in the relative speed of a fluid around a body is accompanies by a decrease in the pressure.
Therefore, a rotating body in a fluid stream experiences a force perpendicular to the motion of the fluid because of the unequal relative speed of the fluid across its surface. It is well known that light has a constant speed irrespective of the relative motion.
Does a rotating body immersed in a stream of photons experience a Bernoulli-like force?
We show that, indeed, a rotating dielectric cylinder experiences such a lateral force from an electromagnetic wave.
In fact, the sign of the lateral force is the same as that of the fluid-mechanical analogue as long as the electric susceptibility is positive (ε>ε0), but for negative-susceptibility materials (e.g. metals) we show that the lateral force is in the opposite direction.
Because these results are derived from a classical electromagnetic scattering problem, Mie-resonance enhancements that occur in other scattering phenomena also enhance the lateral force.
Read more at http://arxiv.org/pdf/1305.0317v2.pdf

Reab also: Optical Bernoulli Forces Could Steer Objects Bathed in Light, Say Theorists

Metamaterials and the mathematical Science of invisibility

Principle of cloaking : (left) A point source radiating in an homogeneous isotropic medium; (middle) A point source radiating in an homogeneous isotropic medium in the  presence of an infinite conducting F-shaped scatterer; (right) A point source radiating in  an homogeneous isotropic medium in the presence of an infinite conducting F-shaped  scatterer surrounded by an invisibility cloak (an anisotropic heterogeneous ring)

Principle of cloaking : (left) A point source radiating in an homogeneous isotropic
medium; (middle) A point source radiating in an homogeneous isotropic medium in the
presence of an infinite conducting F-shaped scatterer; (right) A point source radiating in
an homogeneous isotropic medium in the presence of an infinite conducting F-shaped
scatterer surrounded by an invisibility cloak (an anisotropic heterogeneous ring)

Andre Diatta, Sebastien Guenneau, Andre Nicolet, Frederic Zolla
A review of some recent developments in the field of photonics: cloaking, whereby an object becomes invisible to an observer, and mirages, whereby an object looks like another one (say, of a different shape). Such optical illusions are made possible thanks to the advent of metamaterials, which are new kinds of composites designed using the concept of transformational optics. Theoretical concepts introduced here are illustrated by finite element computations…..
Read more: http://arxiv.org/ftp/arxiv/papers/1212/1212.5408.pdf

Invisibility cloaking in ‘perfect’ demonstration

The trick included developing a diamond-shaped cloaking region – invisible only from one direction

By Jason Palmer

Scientists have succeeded in “cloaking” an object perfectly for the first time, rendering a centimetre-scale cylinder invisible to microwaves.

Many “invisibility cloak” efforts have been demonstrated, but all have reflected some of the incident light, making the illusion incomplete.

A Nature Materials study has now shown how to pull off the trick flawlessly.

However, the illusion only works from one direction and would be difficult to achieve with visible light.

The idea of invisibility cloaking got its start in 2006 when John Pendry of Imperial College London and David Schurig and David Smith of Duke University laid out the theory of “transformation optics” in a paper in Science, demonstrating it for the first time using microwaves (much longer wavelengths than we can see) in another Science paper later that year.

The papers sparked a flurry of activity to move the work on to different wavelengths – namely those in which we see.

As the “Where’s my cloak of invisibility?” article points out The field has moved on considerably since then.

But no effort to date has been able to achieve the “perfect” cloaking that the theory originally described.

The structures that can pull off this extraordinary trick of the light are difficult to manufacture, and each attempt has made an approximation to the theoretical idea that results in reflections.

So someone would not see a cloaked object but rather the scene behind it – however, the reflections from the cloak would make that scene appear somewhat darkened.

Now, Prof Smith and his Duke colleague Nathan Landy have taken another tack, reworking how the edges of a microwave cloak line up, ensuring that the light passes around the cloak completely with no reflections.

The trick was to use a diamond-shaped cloak, with properties carefully matched at the diamond’s corners, to shuttle light perfectly around a cylinder 7.5cm in diameter and 1cm tall.

“This to our knowledge is the first cloak that really addresses getting the transformation exactly right to get you that perfect invisibility,” Prof Smith told BBC News.

However, the cloaking game is always one of trade-offs; though the illusion is perfect, it only works in one direction.

“It’s like the card people in Alice in Wonderland,” Prof Smith explained. “If they turn on their sides you can’t see them but they’re obviously visible if you look from the other direction.”

The design principles that make the cloak work in microwaves would be difficult to implement at optical wavelengths. But microwaves are important in many applications, principally telecommunications and radar, and improved versions of cloaking could vastly improve microwave performance.

“The cloak we demonstrated in 2006 as a kind of microwave device would be very poor, but this one gets us to something that could be potentially useful,” Prof Smith said.

“I think it’s something that a lot of people can build on. Everything in this field is going to come down to what you can make, what you can design. And I think this steps up the design.”

Read more: www.bbc.co.uk

A Double Green Flash

Click to Enlarge

At sunset, the sky is often painted with an array of oranges, reds and yellows, and even some shades of pink. There are, however, occasions when a green flash appears above the solar disc for a second or so. One such occurrence was captured beautifully in this picture taken from Cerro Paranal, a 2600-metre-high mountain in the Chilean Atacama Desert, by ESO Photo Ambassador Gianluca Lombardi. Cerro Paranal is home to ESO’s Very Large Telescope.

The green flash is a rather rare phenomenon; seeing such a transient event requires an unobstructed view of the setting (or rising) Sun and a very stable atmosphere. At Paranal the atmospheric conditions are just right for this, making the green flash a relatively common sight (see for example eso0812). But a double green flash such as this one is noteworthy even for Paranal.

The green flash occurs because the Earth’s atmosphere works like a giant prism that bends and disperses the sunlight. This effect is particularly significant at sunrise and sunset when the solar rays go through more of the lower, denser layers of the atmosphere. Shorter wavelength blue and green light from the Sun is bent more than longer wavelength orange and red, so it appears slightly higher in the sky than orange or red rays from the point of view of an observer.

When the Sun is close to the horizon and conditions are just right, a mirage effect related to the temperature gradient in the atmosphere can magnify the dispersion — the separation of colours — and produce the elusive green flash. A blue flash is almost never seen as the blue light is scattered by molecules and particles in the dense blanket of air towards the horizon.

The mirage can also distort the shape of the Sun and that of the flash. We see two bands of green light in this image because the weather conditions created two alternating cold and warm layers of air in the atmosphere.

This stunning photo was taken by ESO Photo Ambassador Gianluca Lombardi on 28 March 2011. The phenomenon was captured on camera as the Sun was setting on a sea of clouds below Cerro Paranal.

Credit: G. Lombardi/ESO