Effect of Distance in Förster Resonance Energy Transfer
This Demonstration illustrates the concept of Förster resonance energy transfer (FRET). FRET, often referred to as a spectroscopic ruler, can be applied to dye pairs, exploiting spectral overlap as a method of determining distance in a biological system. The process involves incorporating dyes into two particles and then exciting one particle (acceptor) by energy transfer from the other particle (donor). The brightness of the acceptor provides an estimate of the distance between the two particles. The distance slider allows you to observe how the brightness of the two particles (indicated by the radius and color of the spheres) is affected by changing the distance, given in Å. You can also determine the distance at which the efficiency of transfer equals 0.5. This is unique to each dye pair. The efficiency of transfer is also known as the Förster distance.
Snapshot 1: at short distances, the acceptor is much brighter than the donor
Snapshot 2: at the Förster distance, the chromophores are of near-equal brightness
Snapshot 3: at larger distances, the donor is much brighter than the acceptor
 K. C. Suddala and N. G. Walter, "Chap. 15—Riboswitch Structure and Dynamics by smFRET Microscopy," Riboswitch Discovery, Structure and Function (D. H. Burke-Aguero, ed.), Waltham, MA: Academic Press, 2014 pp. 343–373. doi:10.1016%2 FB978-0-12-801122-5.00015-5.
 R. Roy, S. Hohng and T. Ha, "A Practical Guide to Single-Molecule FRET," Nature Methods, 5(6), 2008 pp. 507–516. doi:10.1038/nmeth.1208.