Lighthouses are tall towers with lights at the top that sit on the edge of large bodies of water. Lighthouses inform ships of the location of land during bad weather conditions and if it is safe to approach land. Many lighthouse are still fully functional. Historically, they used small flames with reflected glass for light, but most have changed to electric light in different colors. Lighthouses use different light colors for several reasons.
Characteristic
The most common reason for a lighthouse to have flashing red and white lights is to distinguish it from other lighthouses. The specific pattern of red and white light, as well as the time interval between the flashes, is called the lighthouse's characteristic. In areas with treacherous waters, where there are multiple lighthouses, the characteristic of each lighthouse will inform the ship of its exact location. Some lighthouses only flash white, some flash red and some alternate between the two.
Port
In areas with narrow bays or ports, lighthouses use red and green lights to distinguish the side of the entrance. In navigation, red lights are associates with the port side, or left side, and green is associated with the right. Upon entering a port, the ship will see the red light on the left.
Danger
Occasionally, lighthouses will use red light to notify incoming ships of danger. They will use a flashing red light or switch between red and white to distinguish the warning from their normal characteristics. Red traditionally means stop, and informs the ship to seek out further information.
Visibility
In areas where the port is surrounded by many buildings, lighthouses use colored light to distinguish themselves from the surrounding white lights. At night, it is difficult to differentiate between multiple white lights, so lighthouses use their characteristics for easier navigation.