單向透視鏡不再神秘

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 doesn’t. A one-way mirror has a reflective coating applied in a very thin， sparse layer—so thin that it’s 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 doesn’t 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 mirror’s 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 criminal’s side， the criminal sees his own reflection. On the detectives’ side， the large amount of light coming from the criminal’s side is what they see. In many ways， it’s 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.

幾乎每個人都在電視節目或電影里看到過一名犯罪嫌疑人在被審訊時，警探們會在單向透視鏡后觀察其表現。一塊玻璃怎么能同時做到一側反射光線而另一側讓光線暢通無阻地通過呢？

答案是，其實它無法兩者兼顧。單向透視鏡的表面有一層非常薄且稀疏的反射膜——薄到可以稱之為“半鍍銀表膜”。取這個名字是因為反射分子在玻璃上的分布實在太稀疏了，它的數量僅達到完全不透明的正常鏡子上的反射分子的一半。而且在分子層面上，反射分子均勻地分布，形成均勻的反射薄膜，但它實際只覆蓋了鏡子一半的面積。這層半鍍銀表膜會將照射到其表面上一半的光反射出來，同時讓另一半的光直射過去。

那么為什么犯罪嫌疑人看不到在相鄰屋子里的警探呢？其中的奧秘就在兩間屋子的照明之中。在那間玻璃看起來是鏡子的房間里，光線非常地強，于是就有足夠的光線可以被鏡子表面反射回來。而在另一間玻璃看起來像窗戶的屋子里，要保持昏暗，以保證穿過玻璃射到另一間屋里的光線盡量少。

犯罪嫌疑人所在的房間里，他看到的是自己在鏡子中的影像。警探在另一個房間則看到大量來自犯罪嫌疑人屋里透射過來的光線。從各角度來看，這類似于人們在一間屋子里輕聲細語，而相鄰的一間屋子里卻充滿了嘈雜的音響聲。輕聲說話的聲音可能會傳到隔壁音響聲音大的屋子里，但是它肯定會被巨大的音樂聲淹沒掉。……