Geddes Prize

Darren J. White


In the past I have always been satisfied with the photographs I could produce with a simple 35mm camera. Recently though, I realised that my 35mm camera is rather inadequate, especially for close up work. Thus I decided to invest in a digital camera (a Toshiba PDR-M70 digital still camera, with 3.37 million pixel, 2048x1536 resolution and a large aperture, F=2 - 2.5, 6x zoom lens). This turned out to be a brilliant purchase, opening up a whole new world of possibilities. The only problem with the digital camera is that due to the fact you can view your photographs instantly, I have become more critical of the pictures I take.

 I would like to express my gratitude to my younger brother Duncan, who patiently helped me as I took hundreds of pictures, while in search of the perfect photograph!

 The photographs in this portfolio have not been changed or enhanced by computer in any way.


Thin film interference Soap1 Soap2 Soap3 Soap4

These four pictures were the most difficult to take, due to the fact that the thin (soapy) film had a tendency to disintegrate. For the fringes to appear bright I found (after much experimenting) that a very dark background was needed, but the fringes needed to be illuminated by a very bright, non-direct light at a certain angle. This was no mean feat.

An old television aerial that was cut apart and soldered back together was used to hold the thin film. The pictures were taken in a dark room with a piece of black card as a background. The thin film was illuminated by white light reflected off a white wall by a 200-watt spotlight.

The regular fringe pattern of the picture on page one indicates that the thin film is of a very uniformly increasing thickness. This picture was obtained by capturing the image very quickly before gravity had the chance to alter the thickness of the film very much (gravity tends to increase the film's thickness towards the bottom). The increasing thickness of the film results in constructive interference occurring for a given wavelength at various positions along the vertical. The dark patch at the top of the fringes indicates that the film is too thin for constructive interference (at visible wavelengths) to occur.

The psychedelic picture soap2 was obtained by blowing lightly on the thin film, to disrupt its surface and give an irregular interference pattern. Lighting the hand and the background and taking the picture slightly out of focus enhanced the effect. Soap4 is a similar photograph that is sharply focused. The picture soap3 is a close up of the thin film, clearly showing some surface features.

For more information on thin film interference (and inferior photographs!) see Wolfson and Pasachoff pages 957-960.


Double-slit interference

In 1802 when Thomas Young carried out his famous double-slit experiment, there was no readily available source of coherent, monochromatic light. Now of course we have lasers, thus the interference patterns were not terribly difficult to obtain.

The laser in question was a HL202 Laser Pointer (max output < 5mW, wavelength 670nm). A camera tripod stand was used to hold the laser pen and two holes were punched (0.5mm apart) through the magnetic film from a computer disk. This was held in place by a crocodile clip attached to a metal lamp stand (as shown in the picture below). The film was placed 5mm from the laser aperture, and the film was approximately 8 m from the white screen onto which the interference pattern was projected.



For more information on double slit interference see Wolfson and Pasachoff pages 964-967.


Compact disc

The data stored on a CD is in the form of a series of minute grooves cut into the surface. The depth of these grooves is nearly a quarter of the wavelength of the laser light, which is used to read the data. As the laser light shines onto each groove a path length change of half a wavelength is introduced, causing destructive interference between the incident and reflected beams. While the CD spins the interference is either constructive or destructive and the interfering light collected by the detector is used to produce an electrical signal which is representative of the data on the CD.

These grooves are too small to see with the naked eye, but they lay out spiral tracks which act as a diffraction grating, thus producing the spectrum of colours seen in the photograph.


Water waves

The photographs on pages eight and nine show complex fluid behaviour (which I will not attempt to explain here!) and circular waves on water propagating outwards.

The fluid in the photographs is a mixture of ink and water (so that the images would appear more clearly). The exposure time was 1/1000th of a second and since the camera could not gather many photons for the picture in this time, the fluid had to be illuminated by a 200-watt spotlight. The first photograph was illuminated with white light, while the second photograph was illuminated with blue light (a blue handkerchief was placed over the spotlight).