How wide is 30cm in the rearview mirror?

[Camargo]

In the rearview mirror, 30cm is approximately the width of two fingers. Here are specific techniques for finding 30cm: 1. Observe the position of the hood: After hearing the voice prompt, you need to drive forward and stay to the right. When you see the right front corner of the hood (about 1/3 of its length) align with the roadside line, the distance between the car body and the roadside line is exactly 30cm. 2. Look at the raised part of the wiper: From the driver's seat, when the raised part of the wiper aligns with the right roadside line, the distance is about 30cm. 3. Look at the right front door handle: Observe the right front door handle in the right rearview mirror. If the right front door handle is tangent to the right roadside line, the distance is 30cm; if it's too far, you need to adjust the direction slightly; if it's too close, there's a risk of crossing the line.

[Evangeline]

I've always been curious about how rearview mirrors work. They are mostly designed with a convex surface, which gives you a wider field of view but at the cost of making nearby objects appear smaller. To calculate the apparent width of an object 30 cm away in the rearview mirror, you need to consider the mirror's curvature. For example, the radius of curvature for typical car mirrors is around 1.2 to 1.5 meters. A simple estimate: if the actual width of an object is 10 cm, at a distance of 30 cm, the apparent width would shrink to about half of the original size, because convex mirrors magnify distant objects and compress nearby ones. But this is just a rough estimate—every car's mirror size varies, and SUVs might have slightly wider ones. I've tested it myself: when parking, a box 30 cm behind the car looked like a mini version in the mirror, easily leading to misjudging its size. So in daily driving, don’t rely too much on the mirror for extremely close objects. Turning your head or using the car's built-in camera is more reliable. Safety first—developing good habits can help avoid bumps and scratches.