Abstract: An apparatus and method for measuring optical forces acting on a trapped particle. In one implementation the apparatus and method are adaptable for use in the optical train of an optical microscope that is configured to trap, with a single light beam, a particle suspended in a suspension medium between an entry cover and an exit cover of a chamber positioned on or within the microscope. The apparatus and method involves the use of a single collection lens system having a numerical aperture designed to be greater than or equal to an index of refraction index of the suspension medium intended to suspend the particle in the chamber which is placeable at or near the exit cover of the chamber of the microscope.