The headlights located on the front of vehicles are there to enable drivers to see at night. However, they are also used to notify other drivers of your presence in foggy conditions or a mountain pass where visibility may be very low. In the beginning, headlights were acetylene based flames which had to be manually lit by the driver.
Today, we have both high and low beam functions for different driving conditions, and the headlights can be toggled on or off from inside the passenger compartment by the driver. The switch is hooked into the twelve volt power supply running through the vehicle, and when activated it sends electricity to the headlights, which in turn powers them up.
The headlights on a car should be aimed so that they show the maximum amount of road in front of the vehicle, without being positioned so high that they interfere with the eyesight of drivers traveling the opposite direction. The high beam function of most headlights will provide a beam that travels upward as well as outward, and so should only be used in areas of low traffic for safety reasons. Most of the time headlights can be successfully aimed by positioning the vehicle in front of a wall or other flat surface. In older cars, they can be adjusted from the front of the lens itself by the use of set screws that are used to tilt the headlight in different directions. Most new model cars are aimed via the engine bay from the back of the bulb and need much less calibration when replacing the light.