The existence of an absorption spectrum in light is due to the specific energy levels and transitions of electrons in atoms or molecules. When atoms or molecules are exposed to light, they can absorb photons and undergo electronic transitions, where electrons move from lower energy levels to higher energy levels.
The absorption spectrum arises because atoms or molecules have discrete energy levels. Each electronic transition corresponds to a specific energy difference between two energy levels. When a photon with energy matching that energy difference interacts with the atom or molecule, it can be absorbed, promoting an electron to a higher energy level.
Once the electron is in an excited state, it is not immediately re-emitted as a photon of the same energy. This is because the excited electron will eventually return to a lower energy level, but it can do so through various pathways:
Spontaneous Emission: The excited electron can undergo spontaneous emission, where it transitions back to a lower energy level and emits a photon. However, the emitted photon may not have the exact same energy as the absorbed photon. Energy levels in the atom or molecule can be influenced by various factors, including its environment and interactions with other particles, resulting in a slightly different energy emission.
Non-radiative Transitions: The excited electron can undergo non-radiative transitions, where it releases its excess energy without emitting a photon. This can occur through mechanisms like collisions with other particles or vibrations within the atom or molecule.
Radiative Transitions: In some cases, the excited electron may transition back to a lower energy level by emitting a photon. However, this emitted photon may be in a different direction or with a different energy compared to the absorbed photon, resulting in a scattered or re-emitted photon that may not contribute to the absorption spectrum.
Overall, the combination of various possible pathways for the excited electron to return to a lower energy level, along with factors influencing energy levels within the atom or molecule, leads to the existence of an absorption spectrum. The absorption lines correspond to the specific energies required for electronic transitions, while the absence of re-emitted photons with the same energy is due to the complex behavior of excited electrons and the different ways they can release their excess energy.