Optical Properties: Negative Refraction And Metamaterials

$Optical Properties: Negative Refraction And Metamaterials$

Optical Properties: Negative Refraction and Metamaterials

Negative refraction isn’t something that occurs naturally in nature, it’s something scientists have created using specifically designed materials. So how do they do it?

The index of refraction, n, can be equated to the square root of the material’s relative permittivity times its relative permeability. For most materials, both these values are positive, resulting in the sort of refraction that we’re all familiar with. But, as show in the diagram below, if both the values were negative then the resulting material would have negative refraction

Metamaterials are defined as artificial materials engineered to have properties that have not yet been found in nature, and since negative refraction does not occur naturally, only metamaterials can have negative refraction.

So far, however, metamaterials have only been created that refract microwave and radio frequencies - scientists believe it is unlikely that a material will be created with negative refraction in the visible part of the spectrum.

Sources: 1 2 3 4

Image sources: 1 2 3

(Note: Images 1 and 2 are not actual photographs of negative refraction, but rather depictions of what the effect would look like, given that negative refraction has never been achieved in the visible part of the electromagnetic spectrum.)

Physical Science...In Space!

Each month, we highlight a different research topic on the International Space Station. In May, our focus is physical science.

The space station is a laboratory unlike any on Earth; on-board, we can control gravity as a variable and even remove it entirely from the equation. Removing gravity reveals fundamental aspects of physics hidden by force-dependent phenomena such as buoyancy-driven convection and sedimentation.

Gravity often masks or distorts subtle forces such as surface tension and diffusion; on space station, these forces have been harnessed for a wide variety of physical science applications (combustion, fluids, colloids, surface wetting, boiling, convection, materials processing, etc).

Other examples of observations in space include boiling in which bubbles do not rise, colloidal systems containing crystalline structures unlike any seen on Earth and spherical flames burning around fuel droplets. Also observed was a uniform dispersion of tin particles in a liquid melt, instead of rising to the top as would happen in Earth’s gravity.

So what? By understanding the fundamentals of combustion and surface tension, we may make more efficient combustion engines; better portable medical diagnostics; stronger, lighter alloys; medicines with longer shelf-life, and buildings that are more resistant to earthquakes.

Findings from physical science research on station may improve the understanding of material properties. This information could potentially revolutionize development of new and improved products for use in everything from automobiles to airplanes to spacecraft.

For more information on space station research, follow @ISS_Research on Twitter!

Make sure to follow us on Tumblr for your regular dose of space: http://nasa.tumblr.com

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t-sci-eng

7 years ago

Why Most Metals Are Silver (but Copper And Gold Aren’t)

Why most metals are silver (but copper and gold aren’t)

If we want to understand what gives a metal its color we first need to understand a little bit about the definition of a metal. Metals are materials that experience metallic bonding - wherein the atoms are so close that there is a veritable “sea of electrons” in the substance. (This is also what makes metals conductors, but that’s another story). Basically each atom donates an electron or two that is free to flow throughout the material, unattached to any particular nucleus.

This proximity leads to an overlap in the allowed energy levels of electrons (shown in the lower left hand image above); basically the higher empty electronic levels are so close to the uppermost filled levels (also called the Fermi level) that they form an essentially continuous band of allowed energies.

Now, backtracking a little bit, color in a substance is caused when a material doesn’t absorb a particular wavelength of light. Because of the empty energy levels mentioned above, metals generally can absorb all wavelengths of light in the visible spectrum. This implies that a metal should look black, except that the excited electron can immediately fall back to the state that it came from, emitting exactly the same energy, causing a flat piece of metal to appear reflective. Thus, the reason why most metals are silver. (Also, the flatter a metal, the more reflective, thanks to diffuse vs. specular reflection).

For a few select metals, like copper and gold, the absorption and emission of photons are noticeably dependent on wavelength across the visible part of the spectrum. The graph in the lower right image above shows the reflectance of aluminum, silver, and gold, including wavelengths in the infrared and ultraviolet. Aluminum is pretty reflective overall, and silver is highly reflective in the visible region (about 400 to 700 nm), but gold clearly absorbs wavelengths about 500 nm or below. Thus, it most strongly reflects yellow, giving it its characteristic appearance.

Sources: (first image), 2 (second image), 3 (third image), 4

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7 years ago

These giant mesh nets provide drinking water in the driest desert on Earth.

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7 years ago

Fibonacci sequence in the hiding…

Here’s every total solar eclipse happening in your lifetime. Is this year your best chance?

On Aug. 21, a total solar eclipse will be visible from the continental United States. It’ll be the first to traverse coast to coast in nearly a century. Learn more about past and future eclipses: See full graphic.

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t-sci-eng

7 years ago

Does one of these LEGO men look bigger than the other? They’re actually the exact same size, but are in an Ames room - a false-perspective illusion room that tricks your brain into thinking things are smaller, or larger, than they really are.

You can make one of these models to try this for yourself. Download our free template from here. And it even works in full size, if you can make one large enough!