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2021年
02月06日
16:24 bbbcさん

TED-Ed1381 The most colorful gemstones on Earth   (地球上で最もカラフルな宝石)

  • 英語の話題
  • 英語学習資料
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オパールは宝石の内部で多くの色がきらめく Play of color(遊色効果) がある。
二酸化ケイ素(ガラスの主成分)の小さな球が水と混ざり、長い地質形成で格子を作り、格子の層に応じて光が反射することで起こる。 (10月の誕生石です)

学習用サイト ⇒TED翻訳サイト、 VoiceTube(ログイン不要)

 5分 130wpm            
 
字幕 : 開始後 で字幕On/Off、 で言語選択。文字の色やサイズ゙はオプションから。
.     動画を見るとき、 でフルスクリーンに拡大すると見やすい。

下記英文は ポップアップ辞書(拡張機能) が使えます。

On an auspicious day(吉日) in November of 1986, five Australian miners climbed Lunatic Hill - so named for the mental state anyone would be in to dig there. While their competitors searched for opals at a depth of 2 to 5 meters, the Lunatic Hill Syndicate bored 20 meters into the earth.

And for their audacity(大胆さ), the earth rewarded them with a fist-sized, record breaking opal. They named it the Halley’s Comet opal, after the much larger rocky, icy body flying by the earth at that time.

The Halley’s Comet opal is a marvel, but its uniqueness is, paradoxically, the most usual thing about it. While diamonds, rubies, emeralds, and other precious stones are often indistinguishably similar, no two opals look the same, thanks to a characteristic called "play of color."

This shimmering, dazzling(キラキラと眩しく), dancing display of light comes about from a confluence(合流) of chemistry, geology(地質学), and optics that define opals from their earliest moments, deep underground. It’s there that an opal begins its life as something surprisingly abundant: water.

Trickling(したたる) down through gaps in soil and rock, water flows through sandstone(砂岩), limestone(石灰岩), and basalt(玄武岩), picking up a microscopic compound called silicon dioxide(二酸化ケイ素). This silica-enriched water enters the voids inside pieces of volcanic rock, prehistoric river beds, wood and even the bones of ancient creatures. Gradually, the water starts to evaporate, and the silica-solution begins forming a gel, within which millions of silica spheres form layer by layer as a series of concentric shells.

The gel ultimately hardens into a glass-like material, and the spheres settle into a lattice(格子) structure. The vast majority of the time, this structure is haphazard(無計画な), resulting in common, or potch, opals with unremarkable exteriors.
 common or potch:コモンやポッチの (色のない、オパール固有語)

The tiny, mesmerizing(魅惑的な) percentage we call precious opals have regions where silica beads of uniform size form orderly arrays(配列). So why do those structures produce such vibrant displays? The answer lies in a principle of wave physics called interference.

For the sake of simplicity, let’s look at what happens when a single color of light— green, with a wavelength of 500 nanometers— hits a precious opal. The green light will scatter throughout the gemstone and reflect back with varying intensities— from most angles suffused((満たされる), from some entirely dimmed(薄暗い), and others dazzlingly bright. What’s happening is, some of the green light reflects off of the top layer. Some reflects off of the layer below that. And so on.
 nanometer : ナノメートル (nm、10のマイナス9乗メートル = 10億分の1メートル)
          (nm/m ≒ パチンコ玉(11mm)/地球(1.3万km)、分子1個の大きさ)


When the additional distance it travels from one layer to the next, and back, is a multiple of the wavelength— such as 500 or 1000 extra nanometers— the crests and valleys of the waves match each other. This phenomenon is called constructive interference, and it amplifies the wave, producing a brighter color.

So if you position your eye at the correct angle, the green light reflecting from many layers adds together. Shift the angle just a bit, and you change the distance the light travels between layers. Change it enough, and you’ll reach a point where the crests match the valleys, making the waves cancel each other out— that’s destructive interference. Different colors have different wavelengths, which translates to varying distances they have to travel to constructively interfere.

That’s why colors roughly correspond to silica bead sizes. The spaces between 210 nanometer beads are just right to amplify blue light. For red light, with its long wavelengths, the silica beads must be close to 300 nanometers. Those take a very long time to form, and because of that, red is the rarest opal color. The differences in the arrangements of the gel lattices within a particular stone result in a wide range of color patterns— everything from broad flash to pin-fire to the ultra-rare harlequin.
 オパールの品種名=broad flash、pin-fire、harlequin.

The circumstances that lead to the formation of precious opal are so uncommon that they only occur in a handful of places. About 95% come from Australia, where an ancient inland sea created the perfect conditions. It was there that the Halley’s Comet opal formed some 100 million(1億) years ago.

Which raises the question: in the next 100 million years, silica-rich water will percolate(滲み出る) through the nooks and crannies(隅々) of some of the discarded artifacts of human civilization. What opalescent(オパールのような) plays of light will one day radiate from the things we forget in the darkness?
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