雨過天晴，天邊時常會出現一道彩虹，給人帶來無限的遐想。有人說，彩虹是人間通往天堂的路，有人說，彩虹是女媧補天時留下的彩石。彩虹到底是怎麼形成的呢？就讓下面這篇文章來為你揭開謎底吧！The rainbows appearance is caused by dispersion of sunlight asitis refracted by (approximately spherical) raindrops. The lightisfirst refracted as it enters the surface of the raindrop,reflectedoff the back of the drop, and again refracted as it leavesthedrop. The overall effect is that the incoming light isreflectedback over a wide range of angles, with the most intenselight at anangle of about 40°-42°. This angle is independent of thesize ofthe drop, but does depend on its refractive index. Asseawater hasa higher refractive index than rain water, the radiusof a rainbowin a sea spray is smaller than that of a true rainbow.This isvisible to the naked eye by a misalignment of thesebows.
Since the water is dispersive, the amount that the sunlightisbent depends upon the wavelength, and hence color, of thelightsconstituent parts. Blue light is refracted at a greaterangle thanred light, but because the area of the back of thedroplet has afocal point inside the droplet, the spectrum crossesitself, andtherefore the red light appears higher in the sky, andforms theouter color of the rainbow. Contrary to popular belief,the lightat the back of the raindrop does not undergo totalinternalreflection; however, light that emerges from the back oftheraindrop does not create a rainbow between the observer andthesun. The spectra emitted from the back of the raindrop do nothavea maximum of intensity, as the other visible rainbows do, andthusthe colors blend together and do not form a rainbow.
A rainbow does not actually exist at a location in the sky,butrather is an optical phenomenon whose apparent position dependsonthe observers location. All raindrops refract and reflectthesunlight in the same way, but only the light from someraindropsreaches the observers eye. These raindrops are perceivedtoconstitute the rainbow by that observer. The position of arainbowin the sky is always in the opposite direction of the Sunwithrespect to the observer, and the interior is alwaysslightlybcenterer than the exterior. The bow is centered on theshadow ofthe observers head, or more exactly at the antisolarpoint (whichis below the horizon during the daytime), appearing atan angle ofapproximately 40°-42° to the line between the observershead andits shadow. As a result, if the sun is higher than 42°,then therainbow is below the horizon and cannot be seen as thereareusually not enough raindrops between the horizon (that is:eyeborder=1 Height) and the ground, to contribute. One exceptionis when theobserver is at the top of a mountain or a similarvantage point,for example an aeroplane (see below). Anotherexception occurs whenthe rainbow is produced by a garden sprinkler.Although in thiscase to get sufficient drops they must be verysmall, resulting ina quite colorless bow.
dispersion
: 散射
spherical
: 球面的，球狀的
radius
: 半徑
（實習編輯：顧萍） | |
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