Ok so I understand that an observation mirror becomes transparent if one side is light but what is not clear from the article is that any bright part apparently becomes transparent.
Because I assume the author has a black screen, and white text, causing the white text to shine through the mirror while anything around the text is still black and let's the mirror act as a normal mirror.
Also it would be nice to see which observation mirror he used, since he's Dutch I could probably order the same one to Sweden. Ordering from the US is often a hassle.
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.
Because I assume the author has a black screen, and white text, causing the white text to shine through the mirror while anything around the text is still black and let's the mirror act as a normal mirror.
Also it would be nice to see which observation mirror he used, since he's Dutch I could probably order the same one to Sweden. Ordering from the US is often a hassle.