In fluorescence, the emission peak occurs at a longer wavelength than the excitation peak.

• A. True
• B. False
• C. They are the same
• D. Emission is shorter

Answer: (A)

In fluorescence, after a molecule absorbs light and is promoted to a higher electronic state, it quickly loses some energy through nonradiative processes (like vibrational relaxation) and settles into the lowest vibrational level of that excited state before emitting a photon. Because energy has been dissipated before emission, the photon released has lower energy and therefore a longer wavelength than the one that originally excited the molecule. This shift to longer wavelength from excitation to emission is known as the Stokes shift. That’s why the emission peak is at a longer wavelength than the excitation peak. In practice, this separation helps fluorescence work in practice, allowing filters to cleanly separate the excitation light from the emitted signal. While some fluorophores have small shifts or spectra that overlap, the general principle remains: emission is red-shifted relative to excitation.