How does a car headlight work? The answer is simple. Although a car headlight has only one electric bulb, it can produce two beams of light: the high beam and the low beam. This is possible because the headlight bulb has two electric filaments, which produce two powerful shafts of refulgent energy, for long and short distance.
In the high beam, the light shines sideways in all directions and, uppon hitting the headlight curved reflecting mirror, the light is deflected forward straight ahead, parallel to the optical axis. In the low beam, on the other hand, the light shines sideways in one direction only, thanks to a screenpan; then it is deflected forward when it hits the mirror; however this deflected light direction is not parallel to the optical axis but slightly slantingly to the right side.
Two diagrams clarifying how a car headlight works
