The Rear-View Mirror

Ah the rear-view mirror... such an unassuming but useful object.

I have recently been using the night position a lot, due to the fact that I am driving on sparsely travelled roads in the middle of the Kern County oilfields. We usually travel slower than many cars on the road because our truck (the "logger" not the pickup) is large and sometimes a bit unwieldy. I drive behind the logger in my pickup. This means that people end up following me way too closely on the two-lane roads before getting the opportunity to pass! So at night, flipping the rear-view mirror really comes in handy when these bright lights are right behind me.

I used not to understand how this worked, but it is a simple principle. The mirror has two layers. The outer layer is mostly transparent but partially reflective. The inner layer is reflective and is pointed toward your eyes in the day position. In the bright light of the day, it reflects like a normal mirror, because the outer layer lets most of the light through. The outer layer is set at an angle to the inner layer, the same angle that the mirror rotates when you flip it. In the night position, the light from the car behind you gets mostly transmitted through the outer layer to the inner layer and is reflected away from you toward the ceiling. But because the outer layer is slightly reflective, it reflects just a little of the light back to your eyes, making it much easier to see in front of you but still keep tabs on what's behind you.

If you flip the mirror during the day, you'll notice that all you get is a reflection of the ceiling. This reflection is so bright that it obscures any dimmer reflection coming from the outer layer.

This is the same principle used for two-way mirrors. The two-way mirror both transmits and reflects light. The room on one side is kept very bright, so the people in this room only see the light being reflected from the mirror, which obscures anything being transmitted from the other side. The other side is kept dark so that the people on this side can see the light being transmitted through the mirror without it being obscured by any reflections.

So next time you're driving at night, take a second to appreciate your rear-view mirror!

Welcoming a new addition to my family of Apple products

I am currently writing from a coffeeshop in San Francisco. I have quite a few days off/vacation days, so I drove up here to visit a college friend. I am also writing from my new iPad!! I really like it so far. There are a few things about it that are not ideal, but I still have my old laptop too. Using the iPad all the time makes me think about the physics of touchscreens, so I will give a very brief explanation here. Circuits and E&M were not my strongest parts of physics, so it really will be brief.

From what I have read, the iPhone and iPad use something called capacitive sensing. In the screen there is a layer of conductive material covered by a layer of insulating material. A voltage is applied to the conductive material, creating a static electric field inside the insulating material. When you touch the screen with your finger, your body's ability to conduct electricity will change this electric field. This is why touching the screen with your fingernail or a stylus doesn't work. They are not conductive and therefore cannot alter the field that has been created in the screen. The iPhone and iPad screens are broken up into a grid that can sense touches at multiple points. The changes in the electric fields can be processed to figure out exactly where you are touching and what motions you are making with your fingers.

Thank you Apple for making such good use of this technology!