Stokes shift
/ STOHKS shift /
The Stokes shift is the small downhill step a molecule takes between absorbing light and giving it back. Think of a bouncing ball: you push it up with a certain energy, but friction means it never quite returns to the same height. A fluorophore absorbs a high-energy photon, loses a little of that energy as heat, and emits a lower-energy photon — so the glow always comes out at a longer wavelength than the light that triggered it.
More precisely, the Stokes shift is the gap in wavelength (or in energy) between the peak of a molecule's absorption and the peak of its emission. A blue-absorbing dye, for example, might glow green or yellow; the size of that color gap is the Stokes shift.
It matters because that gap is what makes fluorescence measurable: it lets you filter out the bright excitation light and detect only the shifted glow against darkness. The honest caveat is that a small Stokes shift makes this separation hard — the excitation and emission overlap — so dyes with a large Stokes shift are prized for clean, sensitive detection.
Fluorescein absorbs strongly around 490 nm (blue-green) but emits around 515 nm (green). That roughly 25 nm Stokes shift lets a green filter pass the glow while blocking the blue excitation light.
The wavelength gap between absorption and emission is what makes the glow separable.
Stokes shift always sends emission to longer wavelengths (lower energy). The rarer reverse case, where emission comes out at shorter wavelengths after the molecule borrows heat, is called an anti-Stokes shift.