The photochemical properties of photochromic materials used in transition lenses are based on the reversible reactions of certain organic compounds, such as naphthopyrans, spirooxazines, and chromenes. These compounds undergo a photochemical reaction when exposed to ultraviolet UV light, which is present in sunlight.When the photochromic molecules in the lenses are exposed to UV light, they undergo a photochemical reaction that causes them to change their molecular structure. This structural change results in the absorption of visible light, causing the lenses to darken. The specific reaction involves the transformation of the photochromic molecules from a closed, non-absorbing form to an open, light-absorbing form.When the lenses are removed from sunlight or the UV light source is removed, the photochromic molecules return to their original closed form, which does not absorb visible light. This causes the lenses to become transparent or lighten again. This process is known as thermal relaxation, and it is facilitated by the presence of heat, which helps the molecules return to their original state.In summary, the photochemical properties of photochromic materials used in transition lenses involve reversible reactions of organic compounds that change their molecular structure in response to UV light exposure. This change in structure causes the lenses to darken when exposed to sunlight and lighten when the sunlight is removed.