Water (H2O) is the most abundant compound on Earth's surface. It exists in various states—liquid, solid (ice), and gas (water vapor)—each having unique optical properties. In its liquid form, water is transparent over a broad range of visible wavelengths but absorbs infrared and ultraviolet light. It serves as the basis for many solvents used in optical spectroscopy. Ice, the solid state of water, also has specific optical characteristics like birefringence and is studied for its role in atmospheric optics. Water vapor, on the other hand, can act as a selective absorber of certain wavelengths and is significant in remote sensing applications. Given its ubiquity and importance in life sciences and environmental science, understanding the optical properties of water and its various states is crucial.