An optical tweezer is a device which uses light to impart forces in the fN to pN range. The basic setup involves focussing a laser beam (typically several 10s of mW of power at the output) to a diffraction limited spot with a high numerical aperture optical element. In the focal plane, the laser spot is highly localized and has a high gradient in intensity (ie. as one travels away from the spot centre, the light intensity drops off rapidly). In the dipole limit (ie. for small particles) it turns out that the force felt by an object is proportional to the gradient of the intensity. Since the intensity gradient is always pointing towards the centre of the beam spot, the particle will always be pushed towards the centre of the beam (read "trap").