Q: How do dimmer mirrors work? -Chad D., Roseville

Q:How do dimmer mirrors work? -Chad D., Roseville

A:In the early '80s, my wife (not at the time) was riding with her boyfriend in his Toyota. As they joked and drove, he asked her what that switch was on his own rearview mirror - as if the mirror needed to be turned on to reflect things. She told him it made headlights appear dimmer at night and he burst out, "Come on. What does it really do?"

He hadn't used such a thing - he was new to driving - and when he did, he thought it was magic. It is, though like all magic, there's a trick to it. The trick is simple once described, which is why cars have had this feature for decades.

When you hit that switch on a manual dimmer mirror, you can feel something shift inside. The movement is the wedge-shaped piece of glass constituting the mirror itself. Glass by itself is reflective, as we all know. Look at a clean window during the day, particularly when the sun is shining in, and you not only see through the glass to the outside, you can also see reflected the brightly lit objects in the room.

A rearview mirror with a dimmer switch takes advantage of both of glass' properties - its transparency and reflectiveness. When the switch is in the day position, the silver-colored back portion of the mirror glass is angled to reflect into our eyes what is behind the car. (The thick portion of the wedge is typically at the top, thin portion at bottom.) At night, when you hit the switch, the wedge-shaped piece of glass pivots, bringing the thin edge forward. Now the surface perpendicular to our sight line and reflecting what is behind the car is the front of the glass - not the highly reflective back coating. Because glass reflects less than the highly reflective coating, the headlights reflect more dimly into our eyes.

The back surface of the glass is still reflective, of course, but it is reflecting only the unlit interior roof of the car. That reflection is much dimmer than the headlight beams behind, so our eyes see the lights. If you turn on a light inside the car with the mirror in this position, you will see the headliner more clearly.

Modern technology has brought us other ways to dim our mirrors through compounds that change shape in response to changes in energy, thereby allowing more or less light to reflect toward our eyes.

To investigate just what it is deer whistles are claimed to do, I had called a manufacturer of such a product for last time's column and reported the claims he had made.

One reader emailed to complain that I had gotten information from the product's manufacturer. My point in doing that was to find out the science, or claimed science, behind this product so many of us are aware of in deer country. As it turns out, the owner I got hold of was not the one who had invented the device or started the company; his family had purchased the company from the device's inventor and he did not know the science behind it. I described the theory he offered (that the whistle generates sound heard by deer, which causes them to stop or run away).

Let me add on this subject that the effectiveness of deer whistles is widely debated. Though they have been tried or are in use in many countries, efforts to prove or test just why they are effective, or whether they are, have produced conclusions across the board.