It's not that anything changes in the mirror to make it transparent.
Such mirrors are half transparent, half reflective. If the other side is dark, the reflective effect is much larger then the transparent effect, making it look like a normal mirror.
When there is a light source behind, the light goes through, which means you can see it. The brighter the light, the more you can see what is behind.
Such mirrors are half transparent, half reflective. If the other side is dark, the reflective effect is much larger then the transparent effect, making it look like a normal mirror.
When there is a light source behind, the light goes through, which means you can see it. The brighter the light, the more you can see what is behind.