About 10 years ago, I worked on a project that used triangulation to measure distances with a laser. The specular reflection is falling on a pair of photodiode

About 10 years ago, I worked on a project that used triangulation to measure distances with a laser. The specular reflection is falling on a pair of photodiodes. The relative amounts of light on each diode is indicative of the distance to the specular reflection. A good visual aide that helped me is to imagine a measuring stick laid horizontally across your field of vision, but at a 45 degree angle, such that the left side is closer to you, and the right side is further away. If you abstract your vision to a 2D projection of a 3D volume, you can easily see that as your eyes follow the measuring stick to the left, the distances get smaller, because they actually are closer to you. Conversely, as your eyes follow the measuring stick to the right, the measurements get larger, because they actually are further away. Your brain is just really good at trig.