## INTERFERENCE EXPERIMENTS

Young's experiment

It is not widely known that Thomas Young’s first experiment demonstrating the wave nature of light involved a single barrier rather than two slits. It is described in a paper titled 'Experiments and Calculations Relative to Physical Optics' that was published in the Philosophical Transactions of the Royal Society of London in 1804 (volume 94, pages 1-16). See http://www.jstor.org/stable/107135 . The experiment involving placing a small piece of card into the centre of a narrow beam of light from a small circular hole in a window shade. The card divided the beam into two beams that then interfered with each other

We can make a modern version of this experiment in the following fashion.
The photo below shows how the narrow beam from a laser pointer was divided by a thin piece of stiff wire. I made right angle bend at the very end of a metal guitar string (top E). The adhesive tape on the laser pointer was just to press the on/off button. The divided beam was directed at a white wall about 2 metres away in a darkened room.

WARNING: Never look directly at a laser pointer, or shine it at someone else. Also be careful about 'specular' reflections; i.e. those from mirror–like surfaces.

The distance L from the wire to the screen was measured (2 m in this case). To simplify calculations, an intial photograph was taken with a ruler taped to the wall just below where the interference pattern appeared. The camera and wire should then not be subsequently moved, and the focal length of the camera lens should not be adjusted.

Here are the the interference patterns I photographed using two metal guitar strings: .009 and .010.

Guitar string .009 - specified diameter = 229 μm

Guitar string .010 - specified diameter = 254 μm

The mth maximum should subtend an angle θ given by

d sinθ = mλ

where λ denotes the wavelength and d denotes the width of the occluding object, i.e. the distance between the 'slits'. The angle θ is sufficiently small so that the approximation sinθ = tanθ = θ is reasonable. The slit separation d was then estimated by measuring the distance y between n adjacent minima by comparison with the photograph of the ruler. The following equation was then used

d = nλL/y

The calculated values of y/n were 6.23 and 5.19 mm for the .009 and .010 strings respectively, producing values of 215 and 259 μm. These are in reasonable agreement with the specified diameters of the wires.

This experiment can be used to estimate the diameter of various thin wires, or perhaps hairs, or thin slivers of card.

Two Slit Interference

It is difficult to make a precise double slit. One method I've used involves painting a glass slide with black matt spray paint. I then used a ruler and a sharp knife or scalpel to use make two scratches in the paint as close as possible to each other. Although the edges of these slits are often quite uneven, they still produce an interference pattern.