Abstract
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.