What is the principle of double-sided mirror？

Double sided mirrors and regular mirrors have the same structure: they are both made of transparent glass with a layer of metal coating. The difference between the two lies in the coating: for ordinary mirrors, the coating is basically opaque. After passing through the glass, the light will be reflected back by the coating. When these reflected lights reach the human eye, we see a mirror image. The coating of double-sided mirrors is thinner and semi transparent.

This coating can also reflect light, but about one-third of the light will penetrate the coating and leak to the other side. When one side of the double-sided mirror is bright and the other side is dark, this semi transparent coating will reveal its magic: for people standing in bright places, the light leaking from the dark is too weak and will be ignored by their eyes and brain. What can be seen is only the light reflected back from one's own side, which is the mirror image of oneself. At this point, a double-sided mirror is equivalent to an ordinary mirror. For people standing in the dark, the light reflected back from their own side is too weak and will also be ignored. All that can be seen is the light leaking in from the opposite side, that is, the scene on the opposite side. At this point, the double-sided mirror is equivalent to a transparent glass.

So, the condition for a double-sided mirror to work is not related to how the mirror is arranged, but only to the contrast of light and dark on both sides of the mirror: people in the dark can peek at people in the bright. Once you understand this principle, you can also crack the double-sided mirror: people in bright places can use a flashlight or other strong light to directly shine on the opposite side, making them visible. You can also wrap a small circle on the glass with your hand to reduce the interference of reflected light, and then get closer to observe the opposite side.

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