Sodium (Na) is a soft, silvery-white, highly reactive alkali metal. Due to its high reactivity, it doesn't exist freely in nature and is usually found in the form of salts, like sodium chloride (NaCl or table salt). Sodium reacts vigorously with water, producing heat and forming sodium hydroxide and hydrogen gas. Optically, sodium's emission line in the yellow range (specifically around 589 nm) is well-known. This bright yellow "sodium line" emission is used in sodium vapor lamps, which are a type of gas discharge lamp. These lamps are used in street lighting and give off a characteristic yellow glow. The sodium doublet emission in the yellow range can also be used as a calibration source in spectroscopy.