Ghost Reflections ...
Question: On a long vacation drive, as it gets dark and people are putting on their headlights, you flip the rear-view mirror up into the "night-driving" position. Your daughter notices and asks, "Dad, what did you just do?" You reply, "I flipped the mirror to night-view to reduce the glare from the headlights behind us." She's quiet for a minute and then asks, "Well, how does that work?" What do you tell her?
Answer: Most mirrors are glass with a back reflective coating. This protects the reflective coating from scratches and tarnish. There are front-reflective mirrors but they are rather rare, expensive and usually used in optical systems where you don't want "ghost" reflections from the partially reflecting front glass surface. However, in the case of rear view mirrors, this is exactly what you want to exploit. The trick here is that the two glass surfaces of the mirror are not parallel — the glass is actually a shallow wedge when viewed from the side. When you use the mirror during the day, you have it adjusted so the back silvered surface provides the reflection to your eyes. At night you pivot the mirror, using the factory installed adjuster, by the wedge angle so that the front surface of the glass now directs the reflections to you. Regular clear glass reflects approximately 4% of incident light (in air) from the front surface, so in this position about 4% of the light coming from the headlights behind you makes it to your eyes. The rest of the light (minus losses) is directed up to the headliner of the car (assuming you flip the mirror up to make the adjustment). Pretty simple and beautifully effective.
3 comments:
that doesn't explain how come it works both ways. that is, you can have the mirror be night vision by flipping it up, but you can also do it by flipping it down.
THAT's WHAT I SAID!!! Clearly bull%&$*
Your daughter is going to be so fucked up...
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