Xiaoming went to cram school, and it was dark the day after tomorrow. At the appointed time, he stood by the roadside waiting for his father to pick him up. After a while, he saw his father's car coming from a distance. At this time, careful Xiao Ming suddenly discovered a strange phenomenon: when his father turned the car headlights switch from bright to dark, the light was not parallel, but divergent, just as he imagined. What the hell is this? With the improvement of living standards, many families are equipped with private cars to facilitate travel. I didn't expect that even the headlights of this car contain mathematical principles. Specifically, the parabola principle is playing tricks.

If you pay attention, you will find that the mirror behind car headlights is parabolic. In fact, it is a parabola (a curved surface in three-dimensional space formed by the rotation of a parabola around its axis of symmetry). The light source located at the focal point of the parabolic mirror produces a bright light beam.

Therefore, light is emitted in a direction parallel to the parabolic symmetry axis. When the light dimmed, the light source changed its position. It is no longer focused, and as a result, light no longer travels parallel to the axis. Now the low beam only shines up and down. The upward beam is shielded, so only the downward beam is emitted to a place shorter than the high beam.