单向透视镜不再神秘

Just about1 everyone has seen a television show or movie in which a criminal suspect is questioned while detectives watch from behind a one-way mirror. How does a piece of glass manage to reflect light from one side while remaining clear2 on the other？

The secret is that it doesnt. A one-way mirror has a reflective coating applied in a very thin， sparse layer—so thin that its called a halfsilvered surface. The name half-silvered comes from the fact that the reflective molecules coat the glass so sparsely that only about half the molecules needed to make the glass an opaque3 mirror are applied. At the molecular level， there are reflective molecules speckled all over the glass in an even film but only half of the glass is covered.4 The half-silvered surface will reflect about half the light that strikes its surface， while letting the other half go straight through.

So why doesnt the criminal suspect see the detectives in the next room？ The secret lies in the lighting of the two rooms. The room in which the glass looks like a mirror is kept very brightly lit， so that there is plenty of light to reflect back from the mirrors surface. The other room， in which the glass looks like a window， is kept dark， so there is very little light to transmit through the glass.

On the criminals side， the criminal sees his own reflection. On the detectives side， the large amount of light coming from the criminals side is what they see. In many ways， its the same as if people were whispering in one room while a loud stereo played in the other. The sound of the whisper might carry into the room with the stereo， but it would be drowned out by the intensity of the music.

If the lights in the room with the mirror are suddenly turned out， or the lights in the observation room suddenly turned on， then the one-way mirror becomes a window， with people in each room able to see those in the other. You can see this effect in any mirrored office building at night—if the light is on in an office， you can see into the office just fine.

幾乎每个人都在电视节目或电影里看到过一名犯罪嫌疑人在被审讯时，警探们会在单向透视镜后观察其表现。一块玻璃怎么能同时做到一侧反射光线而另一侧让光线畅通无阻地通过呢？

答案是，其实它无法两者兼顾。单向透视镜的表面有一层非常薄且稀疏的反射膜——薄到可以称之为“半镀银表膜”。取这个名字是因为反射分子在玻璃上的分布实在太稀疏了，它的数量仅达到完全不透明的正常镜子上的反射分子的一半。而且在分子层面上，反射分子均匀地分布，形成均匀的反射薄膜，但它实际只覆盖了镜子一半的面积。这层半镀银表膜会将照射到其表面上一半的光反射出来，同时让另一半的光直射过去。

那么为什么犯罪嫌疑人看不到在相邻屋子里的警探呢？其中的奥秘就在两间屋子的照明之中。在那间玻璃看起来是镜子的房间里，光线非常地强，于是就有足够的光线可以被镜子表面反射回来。而在另一间玻璃看起来像窗户的屋子里，要保持昏暗，以保证穿过玻璃射到另一间屋里的光线尽量少。

犯罪嫌疑人所在的房间里，他看到的是自己在镜子中的影像。警探在另一个房间则看到大量来自犯罪嫌疑人屋里透射过来的光线。从各角度来看，这类似于人们在一间屋子里轻声细语，而相邻的一间屋子里却充满了嘈杂的音响声。轻声说话的声音可能会传到隔壁音响声音大的屋子里，但是它肯定会被巨大的音乐声淹没掉。

如果玻璃看起来是镜子的房间的灯光突然灭了，或者（警探们所在的）观察室的灯光突然被打开，那么单向透视镜就会变成透明的窗户，两个房间的人都能看到彼此。夜晚时，你在任何有玻璃窗户的大楼中都能看到这种效果——如果办公室的灯开着的话，你在大楼外就能把办公室里面看得一清二楚。

注释

1. just about： 几乎，差不多。

2. clear： 畅通的，无阻碍的。

3. opaque： 不透明的。

4. speckled： 布满斑点的，如斑点般散布的；film： 薄层，薄膜。