Bringing the magic of light to remote areas where resources are scarce: Beautiful demonstrations of interference patterns using laser pens and fibres

The training of physics teachers in remote areas in the developing world requires dedicated trainers (who typically are volunteers), as well as robust logistics. The latter must include the supply of equipment for experiments in the classroom. This task is greatly aided by the use of cheap, safe and readily available consumer goods that do not require local power supplies. In this paper, a simple experiment using a laser pointer pen and samples of hair as well as wire and transparent thin fibre is presented, reproducing a variant of Thomas Youngs' famed double slit experiment. The spread of the interference pattern as it projects itself on a screen is sufficiently large to catch the interest of students, and its orientation being perpendicular to that of the hair is also strikingly counter-intuitive. The students are then encouraged to apply the simplified Fraunhofer equation to the various samples to find out the width of their hair. Ideally, these samples would also include calibrating materials like fibres and wires of known diameters, the use of which should give confidence in the model by confirming that it can predict the sample diameter. A fruitful discussion supported by diagrams can also be conducted on the differences that could be expected between a straight edge and a rounded edge, the latter throwing an unexpected challenge to the initial model. However, the use of a transparent fibre also clearly illustrate the limitations of this model, a perception that is amplified by the particularly wide and bright interference pattern that it produces. This mismatch between the model and the real system should prompt the students to further refine their description of the physical system and the resulting model. Throughout the session, their reasoning may be helped by encouraging them to produce diagrams showing the path of optical rays.