You look down and see a yellow pencil lying on your desk. Your eyes, and then your brain, are collecting all sorts of information about the pencil: its size, color, shape, distance, and more. But, how exactly does this happen? The ancient Greeks were the first to think more or less scientifically about what light is and how vision works. Some Greek philosophers, including Plato and Pythagoras, thought that light originated in our eyes and that vision happened when little, invisible probes were sent to gather information about far-away objects. It took over a thousand years before the Arab scientist, Alhazen, figured out that the old, Greek theory of light couldn't be right. In Alhazen's picture, your eyes don't send out invisible, intelligence-gathering probes, they simply collect the light that falls into them. Alhazen's theory accounts for a fact that the Greek's couldn't easily explain: why it gets dark sometimes. The idea is that very few objects actually emit their own light. The special, light-emitting objects, like the sun or a lightbulb, are known as sources of light. Most of the things we see, like that pencil on your desk, are simply reflecting light from a source rather than producing their own. So, when you look at your pencil, the light that hits your eye actually originated at the sun and has traveled millions of miles across empty space before bouncing off the pencil and into your eye, which is pretty cool when you think about it. But, what exactly is the stuff that is emitted from the sun and how do we see it? Is it a particle, like atoms, or is it a wave, like ripples on the surface of a pond? Scientists in the modern era would spend a couple of hundred years figuring out the answer to this question. Isaac Newton was one of the earliest. Newton believed that light is made up of tiny, atom-like particles, which he called corpuscles. Using this assumption, he was able to explain some properties of light. For example, refraction, which is how a beam of light appears to bend as it passes from air into water. But, in science, even geniuses sometimes get things wrong. In the 19th century, long after Newton died, scientists did a series of experiments that clearly showed that light can't be made up of tiny, atom-like particles. For one thing, two beams of light that cross paths don't interact with each other at all. If light were made of tiny, solid balls, then you would expect that some of the particles from Beam A would crash into some of the particles from Beam B. If that happened, the two particles involved in the collision would bounce off in random directions. But, that doesn't happen. The beams of light pass right through each other as you can check for yourself with two laser pointers and some chalk dust. For another thing, light makes interference patterns. Interference patterns are the complicated undulations that happen when two wave patterns occupy the same space. They can be seen when two objects disturb the surface of a still pond, and also when two point-like sources of light are placed near each other. Only waves make interference patterns, particles don't. And, as a bonus, understanding that light acts like a wave leads naturally to an explanation of what color is and why that pencil looks yellow. So, it's settled then, light is a wave, right? Not so fast! In the 20th century, scientists did experiments that appear to show light acting like a particle. For instance, when you shine light on a metal, the light transfers its energy to the atoms in the metal in discrete packets called quanta. But, we can't just forget about properties like interference, either. So these quanta of light aren't at all like the tiny, hard spheres Newton imagined. This result, that light sometimes behaves like a particle and sometimes behaves like a wave, led to a revolutionary new physics theory called quantum mechanics. So, after all that, let's go back to the question, "What is light?" Well, light isn't really like anything we're used to dealing with in our everyday lives. Sometimes it behaves like a particle and other times it behaves like a wave, but it isn't exactly like either.
當你低下頭,望著桌上的黃色鉛筆。 你的眼睛,然後是你的腦袋,便開始接收 各式各樣有關它的資訊: 它的大小、 顏色、 形狀、 與你的距離、 及其他各種特徵。 不過,這一切到底是怎麼發生的? 古希臘人率先開始 以稍加科學的角度思索 什麼是光,視覺究竟又如何運作。 某些希臘哲學家, 包括柏拉圖和畢達哥拉斯在內, 認為光其實源自人的眼睛 而當眼睛發送 肉眼看不見的微小探測器 去收集遠處物體的資訊時,便產生視覺。 直到一千年後 阿拉伯科學家海什木才表示 古希臘的光學理論完全不合邏輯。 海什木認為,人的眼睛根本不會發送 任何迷你情資探測器, 眼睛只是接收 射進來的光線。 海什木的理論 解開了希臘人 「為何有時會一片漆黑」的不解之謎。 「為何有時會一片漆黑」的不解之謎。 理論指出,本身會發光的物體並不常見。 會發光的物體相當特別, 如太陽 或是燈泡, 我們稱之為光源。 我們目光所及絕大部分的東西, 例如桌上的鉛筆, 都只是反射光線 本身並不會發光。 因此,當你望著你的鉛筆 映入眼簾的 其實是鉛筆反射了 穿越百萬英里 源自太陽的光線, 想一想其實還蠻有意思的。 不過,太陽散發出的究竟是什麼東西 我們又如何看得到? 是如原子般的微小粒子? 還是如池塘漣漪般的波? 近代科學家耗時數百年 試圖找出真相。 艾薩克.牛頓是最早開始找尋答案的科學家之一。 牛頓相信 光是由原子般、他稱之為光子的 微小粒子組成。 這項假設足以解釋光的某些特性。 例如折射, 折射是指光由空氣進入水時 所產生的彎曲現象。 不過,在科學界,即使是天才也會犯錯。 牛頓過世很長一段時間後,在十九世紀 科學家們做了一系列實驗 清楚證明 光,不可能由原子般的微小粒子組成。 仔細想想也對,兩道光束交錯時 完全不會互相影響。 如果光是由微小、實心的球體組成, 我們可以預期 A 光束的微小粒子 會與 B 光束的微小粒子產生撞擊。 當兩邊的粒子發生碰撞 粒子則會不規則地四處彈跳。 不過事實並非如此。 兩道光束反而會直接互相穿越, 你可以自行 用雷射筆和粉筆灰實驗看看。 另外,光會有「干涉」的現象。 干涉指的是兩列波在空間中重疊時 產生新的複雜波形的現象。 我們可以在 兩個物體丟進靜止的池塘時, 或是兩道點狀光束放得很近時, 觀察到這種現象。 只有波才會互相干涉, 粒子不會。 更有甚者,把光理解為一種波 便自然而然能夠解釋,什麼是顏色, 又鉛筆為何看起來是黃色的。 結論,光是一種波,對吧? 先別急! 二十世紀時,科學家實驗發現 光亦有一些粒子的特性。 例如,當照在金屬上時 光會將能量由 一種叫「量子」的形式 分批傳遞至金屬中的原子。 不過,我們還是不能忽略如干涉這種波獨有的特性。 這些量子畢竟還是 與牛頓所想像的微小、實心球體有所不同。 這讓光表現得時而像粒子, 時而像波, 使得物理學界產生了革命性的新理論 「量子力學」。 所以,最後還是要回歸最初的問題: 「光是什麼?」 光,其實獨樹一格, 與我們日常生活接觸的事物都有所不同。 有時它的特性像粒子, 有時又像波, 但與兩者 卻都不完全相同。