時差N小時:神圣的影子

In the record of his life at Walden Pond, Henry DavidThoreau reported on an odd phenomenon. "As Iwalked on the railroad causeway," he wrote, "I usedto wonder at the halo of light around my shadow."Thoreau was probably seeing a phenomenon called"heiligenshein," which is German for halo. This is aglowing light around the head and shoulders ofyour shadow. These days, it's likely to be seen byearly morning golfers on dewy grass. Where does thishalo come from?

在瓦爾登湖的生活記錄里，亨利·大衛(wèi)·梭羅報告了一個奇怪的現(xiàn)象。“而當(dāng)我在鐵路堤道上行走的時候，”他寫道：“我常常驚奇地看到我的影子周圍有一個光輪。”梭羅可能看到的是一個叫作"heiligenshein"的現(xiàn)象，"heiligenshein"是德語，意為“光環(huán)”。那是圍繞在頭和肩膀的影子周圍的一圈耀眼的光芒。這些天，清晨在被露水打濕的高爾夫球場上很有可能看見這種現(xiàn)象。這些光環(huán)來自哪里?

Your shadow's halo is made by something called "back scattering." This happens wheneversunlight enters a dewdrop. The sunbeam enters the front of the drop, then a certain amount oflight bounces off the back. This light streams back out the front, almost exactly in the directionit came from. Thanks to back scattering, every dewdrop on the golf course is shining a narrowbeam of light directly back toward the sun.

影子周圍的光環(huán)是由于“反向散射”形成的。當(dāng)陽光射進水滴時，往往會發(fā)生這種現(xiàn)象。陽光從水滴前方進入時，總有一定數(shù)量的光線被反射回去。這些光線從水滴前方反射回去，反射角度幾乎與入射方向一樣。由于反向散射，高爾夫球場上的每一滴露珠都會向太陽發(fā)出一道狹小的光芒。

Heiligenshein is the glare from this backward bouncing sunlight. If every dewdrop bouncessunlight, why does the halo only appear around your shadow's head instead of over the wholefairway? Unlike a light bulb, which shines light in every direction, each dewdrop is reflecting anarrow, tightly focused beam, aimed directly at the sun. Your eyes must be in the path ofthese beams to see them. If you're looking down at your shadow, your eyes are only in the lineof fire for those dewdrops near your shadow's head. The beams from the dewdrops a few feet tothe side miss your eyes entirely. This is why you're likely to see a halo around your ownshadow's head, but not around those of others.

Heiligenshein 就是這種反射光線較耀眼的部分。如果每一滴露水都反射陽光，那么為什么光環(huán)只出現(xiàn)在頭部的影子周圍?燈泡向四面八方發(fā)光，但與燈泡不同的是，露水反射的光只對著太陽，而且反射光線即狹小又集中。只有當(dāng)這些光束射入你的眼睛，你才能看見。當(dāng)你低頭看向影子時，只有影子頭部周圍的露珠反射的光線才會進入你的眼睛，而旁邊幾英尺遠的露珠反射的光芒就會錯過。這就是為什么你可能在影子頭部周圍看到光環(huán)，而不是在影子的其它部位看到。