The position
fluctuations are approximately constant at low frequencies until fc is reached, at which point the slope of the
power spectrum changes significantly. Thus, measurement of the fluctuations in the position of a bead in an
optical trap due to Brownian motion can be used to determine fc. Therefore, from Eq. 13 this is a
measurement of the trap stiffness, k, which allows us to determine the force on a bead in the trap directly
from its change in position using Eq. 4.