The mirror is not ground flat — the front glass surface is at an angle to the back (mirrored) surface. So if you looked at this mirror out of its casing, it would be wedge-shaped with the thicker edge at the top. When you “flip” the mirror, the back mirrored surface actually points toward the dark ceiling, so you don’t see that image. What you see instead is the image reflecting off the front of the glass, and this is much dimmer that the pure reflected image so it does not hurt your eyes.
And now you know! Cool stuff all around us that we never really think of or try to understand, we just use it and never get around to really understanding it. I’ve always thought these mirrors were a super neat invention, sure, they’ve been around for a long time, no less ingeniously simple for their task though!
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