Final answer:
The wavelengths a molecule can absorb correspond to the exact energy differences between its quantized states. When a molecule emits a photon, it releases energy in the form of specific wavelengths that it had previously absorbed to reach the excited state. Emission and absorption spectra are thus fundamentally linked and reversible, revealing structural and energy state information.
Step-by-step explanation:
The wavelengths a molecule can absorb are related to the energy differences between its quantized vibrational or electronic states. When a molecule absorbs a photon of light, its electrons are excited to a higher energy level. The absorbed wavelength corresponds exactly to the energy needed for this transition (given by the equation E = hν, where E is energy, h is Planck's constant, and ν is frequency). Conversely, when a molecule transitions from a higher to a lower energy state, it emits a photon with a wavelength that matches the energy released in this process. Thus, the wavelengths a molecule can emit are the exact wavelengths or energies that it can absorb, given it is returning to a lower energy level from the particular excited state it had previously absorbed energy to reach.
For example, visible light has photon energies ranging from 1.63 to 3.26 eV. When a molecule has an energy difference that matches the energy of a visible light photon, it can absorb that light. For instance, if a molecule absorbs a red photon, it has a precise quantized energy level that corresponds to that red photon's energy. This idea also applies to non-visible light, like infrared, which is frequently involved in vibrational transitions.
Emission spectra are typically complementary to absorption spectra, as seen in experiments with elements like hydrogen. When an electron in hydrogen falls to a lower energy level, it emits a photon with a specific wavelength that corresponds to the energy difference between those two levels. In this way, absorption and emission spectra can reveal detailed information about the structure and energy states of a molecule.