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Electronic transitions occur when electrons in a nonmetallic material absorb energy and jump from a lower energy level to a higher energy level. This process is called absorption, and it causes the material to become less transparent or opaque. When the electron returns to its original energy level, it releases the absorbed energy in the form of a photon, and this process is called emission.

Vibrational transitions involve the absorption of energy by atoms or molecules in a material, causing them to vibrate at a higher energy level. These vibrations can happen in various degrees of freedom, such as stretching and bending, within the molecular structure. When the energy is absorbed, it leads to a change in the vibrational state of the material. As the atoms or molecules return to their original vibrational state, they release the absorbed energy, which can cause the material to change color or opacity.

Rotational transitions take place when a molecule absorbs energy and starts rotating at a higher energy level. This rotational motion can occur around the center of mass of the molecule or along the axes of particular bonds. When a molecule absorbs energy, its rotational state changes, resulting in a shift in the energy levels. As the molecule returns to its original rotational state, it releases the absorbed energy, leading to changes in the material's opacity or color. In conclusion, the three main absorption mechanisms in nonmetallic materials are electronic transitions, vibrational transitions, and rotational transitions. These mechanisms lead to changes in the energy levels of the material's electrons, the vibrational states of its atoms or molecules, and the rotational states of its molecules. These changes can result in variations in transparency, opacity, and color of the material.