FRET stands for fluorescence resonance electron transfer. This phenomenon is observed when two dyes are placed within proper distance of each other (one full turn of DNA is an excellent length). If the first dye is excited and emits photons, and the second dye absorbs that wavelength, the only observed emitted wavelength will be that of the second dye. This ability to see another wavelength when two dyes come into proximity of each other has a number of useful applications for diagnostic assays.