Thanks for your great knowledge. I read your explanation of how a day-night mirror works. The wedge shape makes sense for when you flip the mirror up, the mirrored surface is pointed toward the ceiling and the front glass is vertical, dimly reflecting cars behind. However, I notice that from the day position, I can flip to night by flipping the mirror down. Yes, the mirrored surface is now pointing to the dark seat or floor, but flipping the mirror down means that the glass surface is not pointing even further down - it is not vertical. How does the night dim reflection happen in that instance?
The effect is caused by multiple reflections from the wedged glass/mirror combination.
The primary reflection passes through the glass (99% transmission), bounces off the mirror (95% reflection), and passes back through the glass (99% transmission) = 93% brightness. This is the normal “daytime” use of the mirror.
In the nighttime position, light reflects off the first surface of the glass (1% reflection)= 1% brightness. The angle is offset by twice the wedge angle of the mirror.
But with the mirror pointed downwards, let’s call this the “-nighttime” reflection image: light passes through the glass (99% transmission), bounces off the mirror (95% reflection), bounces off the glass (1% transmission), bounces again off the mirror (95% reflection), and finally passes through the glass (99% transmission) = 0.88% brightness ~= 1%. The angle is offset by (minus) four times the wedge angle of the mirror.
By extension, the “higher order” images that are visible if you keep bending the mirror are explained. In these, the image brightness is so low (1% of 1% for the next image), that only headlights are visible. But I find it useful when the headlights behind me are too bright.
Just a reminder, that column was written back in 1989. Technology may have changed (I personally don’t have a clue), but we don’t have the staff or resources to revise old columns.
