Peephole Reverser
We install peepholes in our doors to protect ourselves, allowing us to identify anyone at the door before we open it to them. But those same peepholes can easily be used against us. A simple gadget called a peephole reverser, also known as a tactical door viewer, was developed by law enforcement, giving them a look at activity inside a dwelling without alerting anyone inside.

逆窥视镜
平常家里的猫眼是一个鱼眼镜头，里面能看到外面的全景，外面看里面就只一个小孔。这个小装置的作用就是就是利用光学原理通过猫眼窥视室内。可见猫眼是不靠谱的。