Turning Pull Into Push? - ScienceNOW

Turning Pull Into Push? - ScienceNOW: To see how this scenario works, consider the case in which just a point charge moves across the surface of an insulator. In that case, the polarization pattern moves with it, becoming so-called evanescent waves that still attract the point charge.

If the point charge moves fast enough, another factor comes into play. In an insulating material such as glass, light travels slower than in empty space. And if a charge moves through the glass faster than light can, it creates a shockwave of light, known as Cherenkov radiation, much like the sonic boom from a supersonic jet. Now, if a point charge above the insulator whizzes along faster than light can within the material, then the induced polarization pattern will move that fast as well and create Cherenkov radiation. That radiation flows at an angle down into the material and carries momentum with it. But by Newton's law that every action has an equal and opposite reaction, the downward flow of momentum must be balanced by an upward push on the point charge.

Soft-Drink Cans Focus Sound Waves to a Point, Beating Diffraction Limit: Scientific American

Soft-Drink Cans Focus Sound Waves to a Point, Beating Diffraction Limit: Scientific American: The group generated audible sound from a ring of computer speakers surrounding the acoustic 'lens': a seven-by-seven array of empty soft-drink cans. Because air is free to move inside and around the cans, they oscillate together like joined-up organ pipes, generating a cacophony of resonance patterns. Crucially, many of the resonances emanate from the can openings, which are much smaller than the wavelength of the sound wave, and so have a similar nature to evanescent waves.

To focus the sound, the trick is to capture these waves at any point on the array. For this, Lerosey and his team used a method known as time reversal: they recorded the sound above any one can in the resonating array, and then played the recording backwards through the speakers...

Normal waves scatter efficiently, so they disappear quickly. However, the evanescent-like waves are less efficient at scattering, and take roughly a second to make it out of the can--a prolonged emission that allows the build up of a narrow, focused spot...

Experiments on wireless power transfer with metamaterials | Browse - Applied Physics Letters

Experiments on wireless power transfer with metamaterials | Browse - Applied Physics Letters: In this letter, we propose the use of metamaterials to enhance the evanescent wave coupling and improve the transfer efficiency of a wireless power transfer system based on coupled resonators. A magnetic metamaterial is designed and built for a wireless power transfer system. We show with measurement results that the power transfer efficiency of the system can be improved significantly by the metamaterial. We also show that the fabricated system can be used to transfer power wirelessly to a 40 W light bulb.