sodiumvapour

Sodium vapour, also written sodium vapor or sodiumvapour, refers to gaseous sodium atoms and ions produced under elevated temperature or during electrical discharge. In spectroscopy, sodium vapour is notable for its strong resonance emission lines near 589 nm, known as the sodium D-lines, corresponding to transitions from the 3p to 3s electronic states. These lines are prominent in hot sodium gas and in low‑pressure discharge tubes.

Production and lighting applications are central to the term. Sodium vapour is produced by heating metallic sodium in an evacuated or sealed tube, often with a starting gas and a ballast to sustain discharge lamps. Sodium‑vapour lamps, including low‑pressure and high‑pressure varieties, emit bright yellow‑orange light with high luminous efficiency; the D‑lines contribute to color rendering. For street lighting, high‑pressure sodium lamps were widely used before the shift to more modern sources such as LEDs.

Other contexts include atmospheric science and astronomy. In Earth’s upper atmosphere, a thin layer of sodium atoms exists in the mesosphere around 90–100 km altitude, largely due to meteoric ablation, and this layer is used in laser guide star experiments for adaptive optics. In laboratory spectroscopy, sodium vapour is used for calibration of spectrometers and for studying atomic structure.

Safety considerations are important. Metallic sodium is highly reactive with water and air, so handling requires strict precautions; in lighting systems, sodium vapour is contained within a sealed bulb under inert gas, reducing exposure risk.