Measuring the diameter of human head hair and 12 gauge wire with a Laser
This experiment aims to find an approximate value for the diameter of a human hair by use of a laser. 12 gauge wire will be used a benchmark as it has a known diameter of 0.12mm.
Published values have shown that human hair can range from 17 to 181µm in diameter (Brian Ley, 1999). As such values from this experiment in this range are likely to be approximately accurate.
This experiment makes use of two ideas in physics, diffraction of waves and superposition of waves. Diffraction of waves is how waves interact when moving through (or around) small spaces (the Physics Classroom, Reflection, Refraction, and Diffraction, 2016), while the superposition of waves is how waves
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This results in an interference pattern which has a minimum at the centre, making the order of all minima an integer value, and thus, the order of all maxima an integer value plus one half. In a double slit experiment a maximum would be at the center, making the order of all maxima an integer value, and thus, the order of all minima an integer value plus one half.
Fg.2 A double slit experiment, which uses the same concepts of diffraction and interference (QIBEC, n.d.)
A laser was used as the light source as it is monochromatic (one colour, ie. one wavelength) and coherent (in phase, ie. crests/troughs align) light. Light can’t be coherent without being monochromatic as the crest/troughs won’t align consistently(see Fg.3).
Fg.3 The different wavelengths of visible light (Randy M. Russell, Windows to the Universe,
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The data collected was reasonably precise (distance from central minimum was consistent across tests) for marking the minima by eye with an error of parallax. Six tests for both the wire and the hair were performed to reduce the impact of random error caused by marking the centre of the minima by hand.
An error of parallax was caused from measuring the centre of the minima from the side of the experiment, instead of perpendicularly. This was because standing in front of the test would block the interference pattern, rendering it impossible to record results. This meant the centre of the minima had to be marked from the side of the experiment instead of perpendicular to it. This decreased the accuracy because the marked centre of the minima was likely not the true centre of the minima. The parallax error is a form of random error as it does not affect all points of data