We present a remotely controlled experiment in which liquid droplets are levitated by a vertically aligned focused laser beam. The droplets levitate at the point where the photon pressure of the focused laser beam balances the gravitational force. The size of a trapped droplet can be measured by detecting the diffraction pattern created by the trapping laser light. The charge on the trapped droplet can thereafter be determined by observing its motion when a vertically directed electrical field is applied. This experiment allows a student to study many fundamental physics processes, such as photon pressure, diffraction of light, or the motion of charged particles in electrical fields. The complexity of the experiments and the concept studied make this suitable for advanced studies in physics. The laser power required in the experiment is about 1 W, which is a thousand times greater than the value of 1 mW at which lasers begin to be capable of causing harm to eyes; high voltages are also used. Further, the cost of the equipment is relatively high, which limits its availability to most undergraduate teaching laboratories. It thus constitutes an ideal experiment for remote control.

We describe an experimental system based on optical levitation of an oil droplet. When combined with an applied electric field and a source of ionizing radiation, the setup permits the investigation of physical phenomena such as radiation pressure, light diffraction, the motion of a charged particle in an oscillating electric field, and the interaction of ionizing radiation with matter. The trapping occurs by creating an equilibrium between a radiation pressure force and the force of gravity. We have found that an oil droplet can be trapped for at least nine hours. The system can be used to measure the size and total electric charge on the trapped droplet. The intensity of the light from the trapping laser that is scattered by the droplet is sufficient to allow the droplet to be easily seen with the naked eye, covered by laser alignment goggles. When oscillating under the influence of an ac electric field, the motion of the droplet can be described as that of a driven, damped harmonic oscillator. The magnitude and polarity of the charge can be altered by exposing the droplet to ionizing radiation from a low-activity radioactive source. Our goal was to design a hands-on setup that allows undergraduate and graduate students to observe and better understand fundamental physical processes.

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