This is a cartoon diagram of the concept of Förster resonance energy transfer (FRET). "A," Two hypothetical flourophores that fluoresce at two different wavelengths. "B1," When these flourophores are mixed at low concentrations there is little FRET (low FRET efficiency) because the distance between flourophores is high. "B2," At high concentrations the distance between flourophores is small resulting in high FRET efficiency. "C," Diagram demonstrating the mechanism of FRET.