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.

幾乎每個(gè)人都在電視節(jié)目或電影里看到過(guò)一名犯罪嫌疑人在被審訊時(shí)，警探們會(huì)在單向透視鏡后觀察其表現(xiàn)。一塊玻璃怎么能同時(shí)做到一側(cè)反射光線而另一側(cè)讓光線暢通無(wú)阻地通過(guò)呢？

答案是，其實(shí)它無(wú)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(chēng)之為“半鍍銀表膜”。取這個(gè)名字是因?yàn)榉瓷浞肿釉诓Ａ系姆植紝?shí)在太稀疏了，它的數(shù)量?jī)H達(dá)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(shí)際只覆蓋了鏡子一半的面積。這層半鍍銀表膜會(huì)將照射到其表面上一半的光反射出來(lái)，同時(shí)讓另一半的光直射過(guò)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(lái)是鏡子的房間里，光線非常地強(qiáng)，于是就有足夠的光線可以被鏡子表面反射回來(lái)。而在另一間玻璃看起來(lái)像窗戶(hù)的屋子里，要保持昏暗，以保證穿過(guò)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(gè)房間則看到大量來(lái)自犯罪嫌疑人屋里透射過(guò)來(lái)的光線。從各角度來(lái)看，這類(lèi)似于人們?cè)谝婚g屋子里輕聲細(xì)語(yǔ)，而相鄰的一間屋子里卻充滿(mǎn)了嘈雜的音響聲。輕聲說(shuō)話(huà)的聲音可能會(huì)傳到隔壁音響聲音大的屋子里，但是它肯定會(huì)被巨大的音樂(lè)聲淹沒(méi)掉。

如果玻璃看起來(lái)是鏡子的房間的燈光突然滅了，或者（警探們所在的）觀察室的燈光突然被打開(kāi)，那么單向透視鏡就會(huì)變成透明的窗戶(hù)，兩個(gè)房間的人都能看到彼此。夜晚時(shí)，你在任何有玻璃窗戶(hù)的大樓中都能看到這種效果——如果辦公室的燈開(kāi)著的話(huà)，你在大樓外就能把辦公室里面看得一清二楚。

注釋

1. just about： 幾乎，差不多。

2. clear： 暢通的，無(wú)阻礙的。

3. opaque： 不透明的。

4. speckled： 布滿(mǎn)斑點(diǎn)的，如斑點(diǎn)般散布的；film： 薄層，薄膜。