fluorescence
Summary
Fluorescence is the emission of a photon when an excited electron drops back to the ground state within nanoseconds of absorbing light. The emitted light typically has lower energy and a longer wavelength than the absorbed light, because some energy is first lost as heat.
Detail
Meaning: A molecule absorbs a photon, and an electron moves to a higher energy level. Some energy is first lost as heat through vibrational relaxation. The electron then falls to the ground state and emits a photon within about nanoseconds, so the glow stops almost as soon as the excitation source is removed. That photon has less energy than the absorbed one, so it typically has a longer wavelength (a Stokes shift).
Equation: E = hf = hc/λ, where E is photon energy (J), h is Planck's constant (6.63 × 10⁻³⁴ J·s), f is frequency (Hz), c is the speed of light (3.00 × 10⁸ m/s), and λ is wavelength (m). Eemitted < Eabsorbed, so λemitted > λabsorbed.
On the MCAT: Passages describe fluorescent dyes or tagged proteins viewed under a microscope. You predict the emitted color (for example, UV or blue excitation giving green emission) or explain why the emission peak sits at a longer wavelength than the excitation peak on a spectrum.
Common confusion: Fluorescence is not phosphorescence. Phosphorescence involves a spin change to a triplet state, and the return to the ground state is spin-forbidden, so emission is far slower and the glow persists after the light is off. Fluorescence is also not reflection: the emitted photon is a new photon with different energy.
Sources
- Fundamentals of Physics
- Physics for Scientists and Engineers
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.