  Distribution of fluorescence resonance energy transfer (FRET) efficiency
between the two ends of a Lennard-Jones polymer chain both at equilibrium and
during folding and unfolding has been calculated, for the first time, by
Brownian dynamics simulations. The distribution of FRET efficiency becomes {\bf
bimodal} during folding of the extended state subsequent to a temperature
quench, with the width of the distribution for the extended state broader than
that for the folded state. The reverse process of unfolding subsequent to a
upward temperature jump shows different characteristics. The distributions show
significant viscosity dependence which can be tested against experiments.
