Sodium vapor refers to the gas formed when sodium metal is heated to high temperatures, causing it to vaporize. This vapor emits a bright yellow light when an electric current passes through it, making it useful in various lighting applications, particularly in streetlights. The distinct yellow hue is due to the specific wavelengths of light emitted by sodium atoms. Sodium vapor lamps are commonly used for outdoor lighting because they are energy-efficient and have a long lifespan. These lamps operate by ionizing the sodium vapor, which produces light. They are often seen in urban areas, providing illumination for roads and public spaces.