11.04.09. An optical trap (also called "laser tweezers") is an experimental setup in which the motion of a small (on the order of a micron in diameter) plastic bead is controlled by a laser beam. The refraction of light through the bead exerts an attractive force (on the order of several piconewtons) on the bead directed toward the center of the beam. Optical traps are useful for a variety of biophysical applications in which one wishes to move or manipulate a single molecule. For example, when the bead is affixed to the cargo end of a motor protein, the laser can be used to control the force against which the protein pulls, which can affect the protein's step frequency or step size (see previous posts). As a visual aid to the lessons below, students were presented with this video, taken by Columbia physics majors Dan Amrhein and Alex Kaz, which shows a fixed laser beam (lower-left corner, directed into the screen) alternately "trapping" and losing a 2-micron bead as the microscope stage is moved from side to side.
