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Landis, a specialist in photovoltaics at NASA's John Glenn Research Center in
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Cleveland, calls predictions of 50% efficiency "reasonable,"
although he dismisses any values higher than that as
"speculative."
Reflection, Refraction, and Beauty
Many attempts have been made, with varying success, to link the concept of
"beauty" to mathematical principles. One such correlation is the index of
refraction for transparent materials; very high values tend not only to bend
and distort the light passing through the material, but also to break it apart
into pleasing rainbows. Absorption spectra, Bawendi would say: you get the
full rainbow, minus the characteristic narrow frequency bands absorbed by the
atoms in the material. Vacuum, by definition, has a refractive index of 1.0,
meaning it does not bend light at all. Air, with an index of 1.0003, is not
much better. I have never heard anyone say that air or vacuum are beautiful
-- not to look at, anyway. But glass has an index of 1.5. You can make
prisms out of it, or little horses or dragons or whatever. Sculpted glass is
widely considered a beautiful commodity. Moving on up, we find leaded crystal
and similar materials, which have been doped to increase their refractive
index. These are more beautiful than glass, while gemstones, with still
higher values, are considered more beautiful still. Diamond, with the highest
refractive index of any natural material -- a whopping 2.4 -- is the most
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beautiful of all.
Interestingly, in 2002 a team of researchers at the Air
Force Research Laboratory in Massachusetts nailed down the far end of the
scale by shining laser beams through a praesodymium-
doped crystal of yttrium silicate, producing a highly excited material capable
of slowing its internal speed of light to zero
-- equivalent to a refractive index of infinity. Light which enters the
crystal simply stops, until excitation source is turned off. Since there's
nothing but blackness to look at when no light exits the crystal, one presumes
the peak of beauty occurs somewhere between diamond's 2.4 and praesodymium
yttrium silicate's infinity.
With their proven ability to manipulate a material's index of refraction,
quantum dots will almost certainly find their way into ornamental objects.
Harvard researchers have even made
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clear plastics into pseudo-gemstones by doping them with nanoparticles of
metal. If such doping characteristics can be varied from one point to another
in a material, it may even be fairly easy to create flat lenses, which mimic
the effect of convex, concave, or fresnel lenses through variations in the
refractive index rather than in the thickness of the lens.
Another way to achieve beauty is to hide things people don't want to see.
Windows are one example: if we can see them, it means they're dirty, which in
turn implies that they're old or ugly or poorly maintained, or that the view
outside is not worth seeing. Most artificial solids have a lower refractive
index than their parent semiconductor, and if these values can be nudged
lower, it might be possible to create "invisible"
objects with optical characteristics similar to air or vacuum.
Still another form of beauty is found in pure metals, which tend to be
lustrous. This is another way of saying that they reflect light very
efficiently, i.e., that a photon striking a metal atom rebounds at a
complementary angle, with little change in its energy. Some metals, like
gold, absorb certain frequencies while reflecting others; this gives them
highly distinctive colors in addition to their luster. A few elements are
good reflectors for all wavelengths in the visible spectrum.
This is especially true when they're molten -- think of a drop of quicksilver
-- but a few of them work well when applied in thin layers to another
material. This is how mirrors are made.
Mercury has a reflectivity ranging from 79% in the ultraviolet to 90% in the
near-infrared. Silver is much better;
its reflectivity is rather poor in the UV, but it reaches 98% in the visible
and 99.5% in the infrared. Aluminum is nearly as good. Other notably shiny
metals include sodium, potassium, and chromium. No mirror is perfect, though
-- at every frequency, at least a little bit of energy is absorbed. When
large amounts of light are being reflected, as in a high-powered laser or
solar oven, the buildup of heat can present major challenges.
It is conceivable that quantum dot materials will provide us with better
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reflectors than any natural atom.
More interestingly, the ability of artificial atoms to hold an asymmetrical
shape may make one-way mirrors a genuine possibility. Today's "privacy glass"
is really just a half-
silvered mirror, designed to reflect 50% of the light that strikes it and
transmit the other 50% through (minus efficiency losses, of course). The
"privacy" effect occurs only when there is a strong difference in illumination
levels from one side of
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the glass to the other. A person standing in a bright room sees
50% of that light reflected back at him; someone in a dark room sees 50% of
his own dark reflection, but it's overwhelmed by the
50% coming in from the bright room. This is why skyscrapers are mirror-bright
by day but seemingly transparent at night, when their internal lighting is
brighter than the night sky. You would not see this effect in a material that [ Pobierz całość w formacie PDF ]

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