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.
你看到下方书桌上有一支黄色的铅笔 然后你的眼睛和大脑 开始搜集关于这支铅笔的各种信息 它的尺寸 颜色 形状 距离 等等 那么我们是怎么做到的? 最早是古希腊人 从科学角度 研究光和视觉的运作机制 包括柏拉图和毕达哥拉斯在内的 古希腊哲学家们 认为光从我们的眼睛发射出来 其中微小无形的探测器 收集到远处物体的信息从而形成视觉 从那以后过了一千多年 阿拉伯科学家阿尔哈曾 才发现古希腊人关于光的理论是错误的 阿尔哈曾认为我们的眼睛并不放出 无形的信息收集探测器 眼睛只用来收集照射过来的光 阿尔哈曾的理论能够解释一个 古希腊人无法解释的现象 那就是:为什么有时候眼前会一片黑暗 理论的核心在于只有少数物体能主动发光 人们把典型的发光物体 如太阳 灯泡 称作光源 大部分我们看到的东西 比如那支桌上的铅笔 仅仅反射了来自光源的光 自身并没有发光 因此你看着铅笔时 眼睛接收到的光实际上来自太阳 光线跨越了无际的太空 才照射到铅笔,随后反射到你的眼睛里 这么一想一定觉得很酷吧 那么太阳发射的光究竟是什么? 我们又是如何看到它的? 它是如同原子一样的粒子 还是如同水面涟漪一样的波? 现代科学家花了数百年时间 才找到答案 艾萨克·牛顿是最早发现答案的一位 牛顿认为 光由一种类似原子的微小粒子组成,并称之为“微粒” 基于这一假设,光的一些属性得到了解释 例如,折射 当一束光从空气射入水中时 它看上去弯曲了 不过,即便是天才科学家也免不了会犯错 牛顿死后过了很久,在19世纪时 科学家做了一系列实验 确切地表明 光不可能是由类似原子的微小粒子组成的 证据在于,当两束光交叉照射时 不会相互影响 如果光的成份是微小的固态粒子 那情况就应该是来自A光束的粒子 撞上来自B光束的粒子 如果真是这样,那相互碰撞的粒子 将会弹向四面八方 然而事实并非如此 实际上,光束会穿过彼此 你自己也可以做个实验 有两支激光笔和粉笔灰就行了 另一个证据就是光有干涉现象 干涉现象是一种复杂的波动现象 当两列波的频率相同时,就会发生干涉 如果两样物体同时触碰平静的水面 就能看到干涉现象 两个点光源距离很近时 也会发生干涉 只有波才有干涉现象 粒子没有 发现光有波的属性之后 自然而然地就能解释颜色是如何产生的 那支铅笔怎么会是黄色的 所以,没错,光就是波 可不能这么快下结论 到了20世纪,科学家从实验中发现 光有粒子的属性 比如,当你向一块金属照射光线时 光间断性地以一种称为“量子”的形式 将能量转移到金属原子中 但光依然有干涉这样的属性 因此光量子并不全然是 牛顿想象地那样微小的固态球体 光有时呈现粒子性 有时又呈现波的属性 开创了一项革命性的物理理论 成为“量子力学” 经过以上分析,让我们回到问题本身 “光是什么?” 光并非我们习以为常的 普通物质 有时它像粒子 有时又像波 用两者任意一方来定义光都不全面