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泊松光斑:阴影中心的亮点

The Brightest Part of a Shadow is in the Middle

Where is the darkest part of a shadow?
阴影中最暗的地方在哪里?
I mean the obvious answer seems to be right in the middle.
我想 “在阴影中心”这个答案似乎显而易见
If you look closely at a shadow,
如果你仔细看这块阴影
as you move the object away from the wall,
当你让物体远离墙面
you notice that the shadow gets a bit fuzzy.
你会发现这块阴影变得有些模糊
So clearly, the edges are lighter.
很明显地 边缘更亮一点
Now, we know that light bends around corners,
现在 我们知道了光在拐角会发生弯曲
that’s a phenomenon call “diffraction”.
这种现象叫“衍射”
So, is it diffraction that’s responsible for the fuzzy edges of shadows?
那么是衍射造成了阴影边缘的模糊吗?
Actually… No.
实际上……不是
The reason that shadows have fuzzy edges,
产生阴影边缘模糊的原因在于
is because most light sources like the sun
绝大多数光源 比如阳光
are not perfectly point objects.
不会正对着某一物体
So, light from one edge of the sun
因此 来自太阳一角的光
is coming in at a very slightly different angle
和形成细微角度差的太阳另一角的光
from light from the other edge of the sun,
会在此处交汇
and that’s what gives us these fuzzy edges.
这便产生了我们看到的模糊边缘
It’s not diffraction.
这不是衍射
But diffraction can play a significant role in shadows,
但是衍射在阴影中有重要作用
and in fact, it was the cause of a massive debate,
实际上 在大概两百年前 这引发了
about 200 years ago.
一场大规模争论
In 1818, the French Academy sponsored a competition
1818年 法兰西艺术院举办了一场比赛
to try to find the best explanation of diffraction.
试图找出衍射的最佳解释
And Augustine Fresnel entered this competition with the suggestion
奥古斯丁·菲涅尔在比赛中提出
that light is a wave phenomenon.
光是一种波动现象
And just like any ordinary wave,
就像任何普通的波浪一样
it bends as it passes around an obstacle.
它在绕过障碍物时会弯曲
But one of the judges was Simeon Poisson,
但作为评审之一的西莫恩·泊松
a harsh critic of the wave theory.
却强烈反对波动理论
He much preferred Newton’s idea that light was a stream of particles.
他更认可牛顿的观点 也就是光是粒子流
And to show just how ridiculous the wave theory was,
只是为了证明波动理论非常荒谬
He showed that Fresnel’s theory would predict a bright spot in the shadow,
他提出 菲涅尔的理论将证明阴影中存在亮点
right in the middle of the shadow behind a circular object.
就在圆形物体背后阴影的正中心
That there would be a spot,
会有一个亮点
almost as bright as if the object wasn’t there at all.
亮到就好像物体根本不在那里
And he thought this was absurd.
他认为这非常荒唐可笑
But my question is,
但我的问题是
Does it exist?
这个亮点存在吗?
Is the brightest part of a shadow really in the middle?
阴影中最亮的部分真的在正中心吗?
To find out, we’re gonna have to do the experiment.
为了找出答案 我们需要做一个实验
So first I had to find some circular obstacles.
那么首先我必须要准备一些圆形物体
I selected spheres instead.
作为代替 我选择了一些球
I got some marbles and some small spherical magnets.
我找到一些弹珠和一些小磁珠
Then for a powerful light source and a large distance to the screen,
接着为了找到一个与屏幕有较远距离的强光源
I selected a lecture theater with a projector.
我选择了一个带有投影仪的阶梯教室
Now the first thing we’re gonna need is a small aperture for the light to pass through.
现在的首要任务 就是找到一个让光通过的小孔
So I’m gonna poke a hole in this card with a screw.
所以我来在这张纸上用螺丝钻一个孔
It’s important to have a small aperture because
如果你想要光处于同相的话
you want the light to be in phase.
小孔是很重要的
And that should work
如果光都从一个小孔中出来
if it’s all coming out of this small little aperture.
那应该就能限制光的相位
But when I put a sphere in front of it …
然而当我把球放在它前面时……
Alright, somebody hit the lights.
好吧 先把灯关上
Here we go…
我们继续……
I can’t see a bright spot.
我无法看到光点
I can’t really see anything.
我什么也看不到
So next I tried a cellphone flashlight, putting a marble in front of that
因此下一步我尝试使用手机上的手电 把一个弹珠放到它前面
And again I could see nothing.
然而我还是什么也看不到
I don’t see anything in the middle.
没有在中心看到任何东西
The idea with Poisson’s spot
泊松亮斑的原理在于
is that light should diffract around a circular object, or a sphere,
光在圆形物体或球的周围会发生衍射
And because the center of the shadow is equidistant from all the edges of that obstacle,
而由于阴影中心与障碍物所有边缘距离相等
all of the light should constructively interfere at that point creating that bright spot.
所有的光线会在这一点发生相长干涉 产生明亮的光斑
So next I tried an overhead projector.
所以下一步我尝试使用高射投影仪
I tried a string of the small spherical magnets.
我试过用一串小磁珠
It’s weird. Like, I feel like I can see it.
奇怪的是 我感觉我可以看到它
Derek: Do you think you can see a bright spot in the middle of that bottom one? Girl: No.
-你能看到最底下的球中心有个亮点吗?-不能
Derek: No?
-不能吗?
I feel like in the very center of each one, I can see a bright spot.
我感觉在每一个的中心 我都能看到一个亮点
I don’t know. My eyes are just totally going nuts up here.
我不确定 我几乎要看花眼了
Like, staring at shadows and trying to see what we want.
就像是紧盯着影子去找我们想要的东西
But this one is opaque.
但是这一个是不透明的
It’s nice and speherical.
它是个漂亮的球体
Set it down…
把它放这……
And if I adjust the focus…
如果我调整焦点……
You can see there’s a bright spot right in the middle of that marble.
你可以看到在那块弹珠的中心有一个明亮的光点
So the spot looks pretty good, but something about it didn’t sit right with me.
尽管这个点看起来非常棒 有些事我还是没办法接受
And you know, when you think something is true,
要知道 当你认为一些事是正确的时候
you really should try as hard as you can to disprove it.
想要驳倒就得用尽全力
What if you used your fingers to kind of make the edges less spherical? Yeah.
如果你用手指让它的边缘不那么圆会怎么样?嗯
You see now that, that I think is concerning, that fact we can still see a bit of spot even though
很显然你现在能看到 实际上我们仍然能够看到一些光点尽管
it looks like you’re…
看起来你……
Right? How is the light getting through and getting there?
对吗?光点是怎么穿过去而又怎么到那的呢?
What was really happening, was light was coming up from
真实的情况是
from the base of this overhead projector, bouncing off this lens,
光从高射投影仪的底部射出 在镜头这里反射
back onto the top of the marble, and then back up through this lens, and onto the wall.
到弹珠顶部后向上射回去 然后又通过镜头 到了墙上
So finally I decided to use a laser
所以最后我决定使用激光
I resisted doing this earlier because in 1818 they didn’t have lasers.
一开始我不愿意这样做 因为他们在1818年的时候并没有激光
In fact, Poisson didn’t do the experiment. He didn’t think that you needed to.
事实上 泊松没有做过实验 他也不认为需要做
The idea of a bright spot in the middle of a shadow was just so ridiculous.
影子中间会有一个光点的想法太荒谬了
And Fresnel didn’t do the experiment either.
而菲涅尔也没有做过实验
But one of the other judges by the name of Arago decided to actually do the experiment.
然而另一位名叫阿拉戈的裁判才真的做了实验
And when he did it, he saw something similar to what I saw.
实验中 他所看到的与我类似
So here I was using the laser through a diverging lens.
所以我让激光穿过凹透镜
And then that beam was shone onto a marble resting on top of a loop of tape.
弹珠放在胶卷顶上 那束光照在它上面
So you can see on the wall, basically exactly what you’d expect.
下面看墙上 一点儿也不出所料
Just a shadow of the marble.
只有弹珠的影子
But when you turn the lights off… there it is…
但是当你把灯关掉……你会看到……
A bright spot right in the middle of that shadow.
在阴影的中间有一个明亮的光点
So the brightest part of a shadow is, in the middle!
所以 只要是一个圆形或球形物体的影子
As long as it’s the shadow of a circular or spherical object.
最亮的地方就是中心!
Now this spot is sometimes called Arago’s spot because he did the experiment and found it.
如今 这个点有时被称为阿拉戈光斑 因为它是阿拉戈做实验发现的
Sometime’s it’s called Fresnel’s birght spot because it was his theory.
有时又被称为菲涅尔亮斑 因为那是菲涅尔的理论
But pretty frequently, it’s referred to as Poisson’s spot.
但是“泊松光斑”这个名字会常见得多
A harsh reminder that it’s not only your great achievements, but also your greatest mistakes
这也在提醒人们 以你的名字命名的东西 不仅可能是你的光辉成就
that can be named after you.
也可能是你最大的错误
We don’t see Poisson’s spot in our day-to-day lives for many reasons,
由于许多原因 我们在日常生活中很少见泊松光斑
one of which is that most objects are not prefect circles.
其中之一就是大多数物体并不是完美的圆形
Plus if they have any surface roughness, really any at all, that will completely wash out Poisson’s spot.
再加上如果它们的表面粗糙 总之任何条件 都可能彻底阻碍泊松光斑的形成
And finally, most light sources are not coherent.
最后 大多数光源都是不相干的
That is, the waves are not coming all in phase: peaks with peaks and troughs with troughs.
换句话说 波形不是完全同相 也就是峰和谷都在同一处的
You might think you could never see Poisson’s spot under ordinary conditions.
你可能认为平常不可能看到泊松光斑
But you can…
但其实你可以……
What you need to do is look at a diffuse source of bright light
你所需要做的就是看着散射的明亮光源
like a fluorescent tube or the blue sky.
像荧光棒或者蓝天
You should see some small light specks drifting around your visual field.
你应该会在视野中看到一些漂浮着的小亮点
Now those are caused by floaters. Little particles actually drifting around inside your eyeball.
那些是由漂浮物造成的 实际上 你的眼球周围漂浮着小颗粒
And they can be all sorts of different shapes, but some of them are spheres.
它们各式各样 但是其中的一部分是球状的
And so they cast a shadow on the back of your retina.
它们会在你的视网膜后投下阴影
And right in the middle of that shadow is Poisson’s bright spot.
泊松光斑就在阴影的最中心
And that is what demonstrated that light really is a wave phenomenon.
而这个现象证实了光就是一种波
And you don’t even have to take my word for it.
你不用非要相信我
You can see it with your very own eyes.
你可以亲眼去看

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视频概述

光与影的艺术使得物理现象充满了魅力,阴影中的一个“不起眼”的亮点居然有这么大的学问,甚至不用工具就可以做一个物理实验?让我们一起来看看吧。

听录译者

收集自网络

翻译译者

Moon

审核员

审核员IBRT

视频来源

https://www.youtube.com/watch?v=y9c8oZ49pFc

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