The mechanism of color change in a photochromic material exposed to UV light involves a reversible photochemical reaction. Photochromic materials contain molecules that can undergo structural changes when they absorb UV light, which leads to a change in their absorption spectrum and, consequently, their color.One of the most common photochromic molecules used in sunglasses and transition lenses is the spiropyran family. In its closed form, spiropyran is colorless and does not absorb visible light. When exposed to UV light, the molecule undergoes a photochemical reaction, transforming into a colored, open form called merocyanine. This open form absorbs visible light, causing the material to darken.When the UV light source is removed, the merocyanine molecules revert to their closed spiropyran form, and the material becomes colorless again. This process is reversible and can occur multiple times without significant degradation of the photochromic material.The photochromic property of these materials can be utilized in practical applications such as sunglasses and transition lenses. In these applications, the lenses are made from a material containing photochromic molecules. When exposed to sunlight or other sources of UV light, the lenses darken, providing protection against bright light and glare. When the wearer moves indoors or away from the UV light source, the lenses return to their clear state, allowing for comfortable vision in lower light conditions.In addition to sunglasses and transition lenses, photochromic materials can also be used in other applications such as smart windows, which can automatically adjust their tint based on the intensity of sunlight, and in various sensors and switches that respond to changes in light conditions.