The color of a dye or pigment changes when exposed to different wavelengths of light due to the absorption and reflection of specific wavelengths by the molecules in the dye or pigment. This phenomenon is governed by the electronic structure of the molecules and their energy levels. The underlying photochemical mechanisms can be explained through the following processes:1. Absorption of light: When a dye or pigment is exposed to light, its molecules absorb specific wavelengths of light corresponding to the energy difference between their ground state and excited state. This energy difference is determined by the molecular structure and the arrangement of electrons in the molecule. The absorbed wavelengths are not visible, and the color we perceive is the result of the remaining wavelengths being reflected or transmitted.2. Excited state and relaxation: Once a molecule absorbs a photon of light, it is promoted to an excited state. The molecule then undergoes various relaxation processes, such as vibrational relaxation, internal conversion, and intersystem crossing, to return to its ground state. These processes can lead to the emission of light fluorescence or phosphorescence or the release of energy as heat.3. Photochemical reactions: In some cases, the excited state of a molecule can undergo a photochemical reaction, leading to a change in the molecular structure. This change can alter the energy levels of the molecule and, consequently, the wavelengths of light it absorbs. This results in a change in the color of the dye or pigment. Examples of such reactions include photoisomerization, photodissociation, and photooxidation.4. Photobleaching and photodegradation: Prolonged exposure to light, especially ultraviolet UV light, can cause the breakdown of the molecular structure of dyes and pigments. This process, known as photobleaching or photodegradation, can lead to a loss of color or a change in the color of the dye or pigment. Photobleaching occurs when the excited state of a molecule reacts with other molecules or undergoes irreversible chemical reactions that alter its structure and absorption properties.In summary, the color of a dye or pigment changes when exposed to different wavelengths of light due to the selective absorption and reflection of specific wavelengths by the molecules in the dye or pigment. The underlying photochemical mechanisms involve absorption of light, relaxation processes, photochemical reactions, and photobleaching or photodegradation.