Potassium (K) is an alkali metal that is highly reactive, especially with water. While potassium itself is not used in optical applications due to its reactivity and lack of transparency, its compounds, like potassium bromide (KBr) and potassium chloride (KCl), are highly useful. These compounds are often used in the form of transparent crystals for the transmission of infrared light in a range of spectroscopic techniques, including infrared spectroscopy. Potassium compounds can also serve as host materials for various dopants in solid-state lasers and optical amplifiers. Potassium's reactivity and solubility often necessitate special handling and storage conditions to maintain optical quality.