Night driving on country roads often becomes a test of vision, especially when a car with poorly adjusted or too bright headlights is driving behind you. While modern, more expensive cars have electrochromic mirrors that darken automatically thanks to special sensors and an electric pulse, the vast majority of cars on the road use a far simpler but ingenious mechanical solution.
At the bottom of the standard interior rear-view mirror is a small lever, the function of which remains a complete mystery to a large number of drivers. Pressing or pulling this thumb changes the angle of the inner mirror element, instantly switching between day and night mode to protect the driver's retina from direct light.
There is precise optical physics behind this simple movement. The new mirror is not just a flat piece of glass with a silver coating on the back, but has a wedge-shaped shape - the front glass surface and the rear reflective layer are not parallel, but form an angle of several degrees. During the day, the mirror is adjusted so that the rear, silver-coated layer reflects about forty percent of the light directly into the driver's eyes.
When darkness falls and the headlights of the car behind begin to dazzle, the lever is switched to change the position of the wedge. With this change, the strong reflection from the rear layer is redirected upwards towards the ceiling of the car, while only about four percent of the light coming from the front glass surface alone is reflected into the driver's eyes. This allows the driver to see the silhouette and the distance to the car behind without his retina being dazzled.
Many drivers avoid using this mechanism because they mistakenly believe that changing the angle will ruin the mirror's adjustment and require readjustment during the day. In fact, the lever is designed with a fixed stroke that returns the mirror to exactly the same daytime position with a single movement. Using this forgotten extra not only reduces eye fatigue during nighttime driving, but also ensures a significantly higher level of road safety.