rear-view mirrors have TWO "night" positions

Comments on Cecil's Columns/Staff Reports
#1

Today’s classic article, http://www.straightdope.com/classics/a4_015b.html , says that the low-light-level reflection off of the center rear-view mirror is a reflection off the front of the glass.

This would mean there should be one, and only one, position the mirror can be in where you can see the headlights behind you dimly reflected.

However, every car I’ve driven has TWO mirror positions that give a dim reflection. One is about one rear-window-height above the main reflective (silvered) viewing angle, and the other is about one rear-window-height below it. I can even adjust the mirror so that the “dim” mirror-switch position is showing me the bright, silvered reflection, and the normal “daytime” position shows me the dim reflection.

What’s causing the second dim reflection? Is the mirror’s reflection getting bounced off the inside of the glass’s front surface, and then bouncing off the mirror again before it reaches my eyes? Or what?

#2

When the light hits the glass some of it (most I should think) is passed through and (some of) the rest is scattered in all directions so there should be an infinite number of positions that you could see a dim reflection from. The strongest images would be the ones that are closest to the real “daylight” angle but if the daylight image and the night image overlaped, the daylight image would overwelm the night image and, on the other end of the range the image would get dimmer as the angle increased. So your best image is just above or below the view of the rear window. I prefer above because there are less distractions on the roof then in the back seat

#3

If that were the case, every pair of headlights should appear in my mirror’s “night” setting as two continuous vertical lines of light that get dimmer the farther away they are from the silvered angle. I do NOT see this. I see one discrete pair of dimly-reflected headlights surrounded by near-total darkness.

And what DO you kids have going on in the back seat that’s so distracting, anyway? :wink:

#4

The rearview mirror has a piece of glass in front of the metallized part of the mirror. In the normal position, the light goes through the glass, reflects off the metallization, back through the glass, and into your eyes. In the intended night vision position, the light reflects off the front surface of the glass, directly into your eyes.

The glass is wedge shaped, so these two reflections aren’t lined up together (the glass in a normal mirror is parallel to the metallization). The second night vision position is when the light reflects first off the metallization, then the glass, then the metallization again. If you tip the mirror farther at night, you can get positions where there are two or more reflections off the glass. These get progessively dimmer.

Since the glass is wedge shaped, it acts as a prism. In the wrong night vision position, car headlights will be bluish tinged at either the top or bottom, and reddish tinged at the other. The dimmer reflections are even more distorted. You can tell the correct night vision position because it won’t have this prism effect.

#5

Ok, Im wrong about the many image thing. I don’t know what I was thinking. You do get (at least) four images: Up, down and to the sides.

As for what goes on in my back seat that is so distracting, perhaps the less said about that the better.

dan

#6

Think about this, you knuckleheads. There are two ways to adjust a rear-view mirror - (a) by using the “night” switch, and (b) by moving the whole mirror with your hand. Using method (b), you can adjust the mirror so it shows the “daytime” view with the night switch forward, and likewise so it shows the daytime view with the night switch back. Either way, you can see the “night” view by flicking the night switch in the opposite direction. No need to resort to complicated explanations involving multiple reflections, etc.

#7

Cecil wrote
> Think about this, you knuckleheads. […]
> No need to resort to complicated
> explanations involving multiple
> reflections, etc.

Tracer wanted to know what was causing the second reflection, and I told her. What’s the big deal?

#8

“Her”? Wow! Does that mean I’m officially female now, without even having to go through Sex Reassignment Surgery? :slight_smile:

But seriously, folks, mblischke’s explanation sounds the most plausible. Yes, Uncle Cecil, I do readjust the mirror by hand so that the “day” (silvered) view appears in the “night” (unsilvered) switch position – I merely noticed that some kind of “night” position is available whenever the “day” position is pointing towards my eyes and I switch the switch to the other position, regardless of whether I’d adjusted things so that the “day” mirror appears in the switch-forward or switch-back position initially. Subsequent investigation (by moving my head up and down) showed that there was, indeed, a “night” reflection above the “day” mirror and another “night” reflection below the “day” mirror.

The reflection off the silver, then off the inside of the glass, then off the silver again seems the most likely explanation for the second “night” image on the opposite side of the mirror from the front-of-the-glass reflection. And no, Danno, I’ve never seen another reflection off to the left or right of the “day” one.

I’m not flying fast, just orbiting low.

#9

Ok,

Let me chime in. I’m what’s known as an expert magnet. I chime in, experts follow with flamethrowers. It’s a life. Anyway, I’m sitting just 3 feet from my old college optics text book Optics by Hecht and Zajac. I looked in it briefly, decided we weren’t talking about Fabry-Perot Etalons and put it away! Since my autos do not possess this kind of rear view mirror, I found one elsewhere. My examination indicates that the mirror I was looking at was constructed out of two pieces of glass. One clear, one mirrored. I didn’t smash the thing with a hammer, (the Pep-boys guys might not have liked that) so this is based on the assumption that I know mould seams when I see them. Anyhow, the construction is like a prism, the two glass pieces together at the top, and apart at the bottom. I should mention that the front piece is clear. Drawing this on a piece of paper, and identifying the angle between the pieces of glass as theta, I found that a ray of light perpendicular to the mirrored back have three reflections. One, reflecting of the mirror and straight out, secondly, off the front surface of the front glass, and lastly (as was suspected by to OP), one that reflects off the mirror, internally is reflected off the front glass, is re-reflected off the mirror, then out into space. I found the angle of reflection off the front of the glass to be twotheta up, I found twotheta down to be the other dim reflection’s angle. This seems to be consistent with your original observation.

P.S. I didn’t consider the internal refraction that occured through the front glass and the rear-mirrored glass. Since the glass is relatively thin I didn’t think it neccessary.

#10

Oops,

Don’t you wish you could edit your own, unreplied posts! For you nit-picker, I know that there is a finite number of addition images created as the light is internally reflected repeatedly. The intensity of these reflections drops off by the reflectance raised to the power of the number of internal reflections.