What is the Stokes shift in fluorescence?

• A. Shift in wavelength from long to short
• B. Shift in wavelength from short to long
• C. No change in wavelength
• D. Change in brightness

Answer: (C)

Stokes shift is the difference between the excitation and emission wavelengths of a fluorescent molecule. After absorbing a photon, the molecule typically relaxes nonradiatively to the lowest vibrational level of the excited electronic state, losing some energy as heat. When it emits a photon as it returns to the ground state, the emitted light has lower energy, so its wavelength is longer than the excitation light. This makes the emission spectrum red-shifted relative to the excitation spectrum. The amount of shift depends on the fluorophore and its environment. So the correct idea is a shift from short to long wavelength; there is a real change in wavelength and it’s not about brightness.