Apple #651: Day/Night Rear-View Mirrors

 I've had a request!  Daily Apple reader Jeroboam wants to know, when you switch your rear view mirror to the night-time view so the reflection from headlights doesn't beam straight into your eyes, how does that work?

I've always suspected mirrors are involved, but I didn't know exactly how, so I thought this was an excellent question.


Most cars' rear view mirrors have a button or a lever at the bottom that allows you to adjust the mirror to reduce the glare from headlights at night.  Pushing that button or tripping the lever does help -- but what exactly is happening with the mirror?  Why does this work?
(Image from Mobile Magazine)

• Lots of people try to explain how this works and I'm sure they do a fine job of it, but the words just aren't sinking in for me.  I need diagrams.  Pictures.

• I found a video of a physics lecture given by Bill Layton of UCLA, and he drew pictures on the blackboard.  Those, I get.

• So I will reproduce for you, using my very rudimentary skills, his diagrams.

• For all you physics purists out there, these diagrams are not to scale, and the angles of reflection are not in any way mathematically measured or anything like that.  They're only meant to demonstrate the general process.

• The first thing to know about your rear view mirror is that it isn't like the mirror you have in your bedroom.  It's not a piece of glass laying flat over a silvered surface.  Your rear-view is made of a piece of glass and it is in front of the silvered surface, but they are at different angles to each other.

In these diagrams, you are sitting to the left of the glass, and the right edge of the silvered mirror is the back of the rear view mirror.
(Diagram by the Apple Lady)

• The fact that the glass and the silvered surface are at angles to each other is why this type of rear view mirror is sometimes referred to as a prismatic rear view mirror.

• The fact that the glass is less reflective than the silvered surface is also crucial to how the whole thing works.  

• When light from the headlights comes streaming into the car and strikes the glass, that light is reflected back off the glass.  But the reflection is relatively weak, so in the daytime, you don't perceive that reflection.

Also in the daytime, the angle of the glass relative to your eyes is such that the reflection bounces off in a direction not aimed at you.  So that's another reason why you don't perceive the reflection of the headlights off the glass.
(Diagram by the Apple Lady)

• The light from the headlights doesn't stop there, of course, but continues on to strike the silvered surface at the back of the rear view mirror.

• That silvered surface gives back a stronger reflection than the glass will (though slightly less strong than the original beam of light), and it bounces back at an angle that's pretty close to the same angle at which it traveled to the mirror. 

• So the headlight reflection that you perceive during the daytime is the reflection that's bouncing back off the silvered mirror.

Here's the whole process in action, during the daytime.  The light from the headlights is bouncing off the glass, but weakly, and at an angle not aimed at you.  The light is also bouncing off the silvered surface, but to a stronger degree than that off the glass, and at an angle aimed closer to your eyes.  So you are perceiving only the reflection bouncing off the silvered surface.
(Diagram by the Apple Lady)

• At night, when you tilt the rear view mirror, you're changing the angle of the glass and the mirror relative to your eyes.  

• When it's tilted for night-time use, the glass is is at the same angle that the silvered surface was.  Now, the glass is angled so its reflection will bounce back toward you, while the mirror will be angled so its reflection bounces away from you.

The day/night rear-view mirror in its night-time orientation.
(Diagram by the Apple Lady)

• Now when the light comes streaming into the car, when it bounces off the glass surface, even though that reflection is weaker, since everything else is darker, you'll be able to perceive that reflection.  And it will be angled toward your eyes.

Even though the reflection off the glass is relatively weak, because it's dark out and there isn't as much light competing with it, you will be able to see that reflection.  Also, the reflection off the glass is now angled in your direction.
(Diagram by the Apple Lady)

• Once again, the light from the headlights doesn't stop when it hits the glass but continues on to the silvered surface.  Now, when it bounces off that, even though that reflection is stronger, it is angled in a different direction, away from your eyes, so you don't perceive it.

Though the reflection off the silvered surface is stronger than that off the glass, it is angled away from your eyes, so that's not the reflection that you see.
(Diagram by the Apple Lady)

• When I tilt the rear view to its alternate angle at night, I've sometimes noticed maybe the ghost of a reflection, or almost two images of headlights.  I've been sort of distracted by that, and I've been curious about why that is.  Now that I now how the rear-view works, I'll see if adjusting the angle of the mirror as a whole helps get rid of that ghost/mirror reflection.

Thanks for asking the question, Jeroboam!

Here's the video of the physics lecture, if you want to see the professor draw and explain the diagrams for you.  The informative stuff starts at about 1 minute 15 seconds in.

You might also be interested in: How Do I Get My Windshield Wipers to Stop Making that Annoying Noise?

Other Sources

Howstuffworks, How does a car's glare-resistant mirror work when it's in the glare-resistant setting?

The Straight Dope, How do "night" rear view mirrors work?

Today I Found Out, How a Car's Rear-View Mirror Works