Optical Tweezers Lab Report

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Almost every fictional story involving the extra-terrestrial includes a type of UFO abducting human with a beam of light. Here is where the concept of the tractor beam was first introduced, and since then, there has been a numerous advances in research striving to make it a reality. Till now it has been successful only on a microscopic scale, which is the topic of discussion of this paper. Optical tweezers are an instrument that use a high-quality microscope to focus a laser beam and in turn hold particles. The force that helps in trapping the particle in light consists of light scattering and gradient forces. These forces are usually sufficient enough to not only allow to trap the particle but also move it small distances. This is usually…show more content…
These specifications were chosen to work along with the particle’s heat absorption distribution. The next part of the experiment is to actually get the laser to trap the particle. The particles were placed in the beam by attaching them on a cylindrical curvet. On rotating this curvet, the particles were detached and fell. On falling, they were trapped by the beam, in the region were the intensity was minimum, i.e the doughnut’s hole. The experiment was setup in a way to make sure that the laser’s axis was always perpendicular with the gravitational force (to simplify calculations). An issue that arises here is beam divergence. It is the increase in the diameter of the beam as its distance from the source increases. This affects us as with increase in divergence, the photophoretic force also increases. To combat this, the velocity was measured also at a distance of 0.05 away from the beam. This resulted in only a 0.1% difference in results. The downwards motion of the particle was given by Stokes Law that says F=6 πa ηV. Using this equation and experimentally obtained error percentages, the magnitude of longitudinal component of photophoretic force can be found. The other component is balanced out by gravitational force. Hence the vector of photophoretic force can be

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