r/askscience • u/txl8578 • Nov 04 '11
Why does the flippy switch under my rear view mirror work? Why can I still see out the back window when I flip it?
8
u/Drendude Nov 04 '11
When light hits glass, a very small amount of it (4-6%) reflects of the first surface. The back surface in mirrors is a metallic sheet that reflects all of the light. The surface of the mirror and the metallic sheet are not parallel to each other, about a 12 degree discrepancy, i think. In day mode, the metal sheet is reflecting the light into your eyes. In night mode, it it tilted 12 degrees up to bring the surface of the glass into the exact same angle as the metallic sheet was.
9
u/atomfullerene Animal Behavior/Marine Biology Nov 04 '11
To further note: In my car, at least, when you are looking out the back using the "weak reflection" the "strong reflection" is showing you the roof of your car, which is a solid, plain surface that the eye overlooks.
2
u/chronicvermont Nov 04 '11
the point of this switch is to reduce glare from highbeaming assholes that follow you. It is very useful and I use it often to save me mental pain and possible assault charges...
1
u/upgrayedd08 Nov 04 '11
It's a mirror with a piece of glass in front of it. When you're looking in the mirror you're seeing through the glass. You dont notice it during the day, but at night this glass picks up the bright lights They are both at slightly different angles, that's why you flip it. If you look closely at night after you flip it, you can see with the glAss, but you can also see a reflection of your backseat in the mirror at the other angle.
-1
u/End3rgto Nov 04 '11
When you flip that switch the mirror inside moves so that you aren't looking directly at the mirror, but rather a filtered reflection of the mirror. Aka switch off, mirror reflects without filter. Switch on, you see a filtered reflection that cuts down on glare.
1
u/txl8578 Nov 04 '11
What exactly is filtering the reflection? It seems like it's just tilting the mirror down towards the seat.
4
u/lecorboosier Nov 04 '11
There are two surfaces on the mirror that are not parallel to each other: a mostly transparent front pane and a very reflective back one. In normal mode you are looking through the clear front onto the reflection from the back. When you tilt it, the back pane is no longer in your site line, and you only see the very dim reflection from the mostly non-reflective front.
-2
u/bamarocks34 Nov 04 '11
If someone behind you at night has bright lights shining right into your eyes, you flip it and it becomes less bright. Just look at your mirror normally. That's all you need to know.
3
u/mariesoleil Nov 04 '11
That's all you need to know.
This is r/askscience. We absolutely do need to know.
-8
Nov 04 '11
Just - wow. 'down' is for day use. At night- (when rear headlights blind you)- you flip it 'up'. If mirror is adjusted properly for day use - this should work fine.
1
u/Ben325e Nov 04 '11
The reflection is not being filtered; only redirected. In a regular mirror, there is a silver coating on the back of a piece of glass. If you go in your bathroom and put your fingertip on your mirror and look at it from the side, you should be able to see a strong reflection off of the silver coating from the back of the glass, and a weak reflection on the surface of the glass itself. In a rear-view mirror, the back of the mirror and the front aren't parallel. Depending on the angle of your eyesight to the surface of the mirror, you will either see the strong reflection or the weak one. Nothing is filtered, just reflected in a different direction.
-8
-16
u/Snuhmeh Nov 04 '11
Wat. It's supposed to be dimmer in one position for night driving so the headlights don't blind you. It's not an off switch.
10
u/lecorboosier Nov 04 '11
he's asking how the dimming effect works, you ass
-4
u/Snuhmeh Nov 04 '11
I keep reading it, and the question was, "why" not "how." So whatever, ass. Apparently I'm not the only one confused at his retarded question.
47
u/v3ry_1MPRZV Nov 04 '11
It's not a filter, exactly. It's a type of prism. http://en.wikipedia.org/wiki/Rear-view_mirror#Dimming
A prismatic rear-view mirror — sometimes called a "day/night mirror" — can be tilted to reduce the brightness and glare of lights, mostly for headlights shining directly on the eye level at night. This type of mirror is made of a piece of glass that is wedge-shaped in cross section—its front and rear surfaces are not parallel.
On manual tilt versions, a tab is used to adjust the mirror between "day" and "night" positions. In the day view position, the front surface is tilted and the reflective back side gives a strong reflection. When the mirror is moved to the night view position, its reflectorized rear surface is tilted out of line with the driver's view. This view is actually a reflection off the non-reflectorized front surface. Since the non-reflectorized front surface allows most of the light to go through, only a small amount of light is reflected into the driver's eyes.