Tom was looking across the St John River at Fredericton, New Brunswick, Canada on 19th May ’11. There was mist or fog lying on the river. A distant sodium light behind him cast his shadow through the fog as a Brocken Spectre. The glory surrounds the shadow of his camera. The image is a 30-second exposure, f/3.5, ISO 400, fl = 18mm, Canon Rebel camera.
Our scientific world view is sometimes coloured by reputation and authority. The 17th Century saw rival hypotheses on the nature of light. To grossly oversimplify the scientific fervour and networking of Europe at those times, there was Isaac Newton’s (1642-1727) concept that light consisted of corpuscles or particles and the opposing view that light was a wave motion.
Diffraction and interference colours were already known by Francesco Grimaldi (1618-1663) and Robert Hooke (1635-1703 ) who proposed “every pulse or vibration of the luminous body will generate a sphere” – the beginning of wave theory. Wave concepts were further developed by Christiaan Huygens (1629-1695). It took quite some contortions to attempt to explain diffraction using corpuscles. In spite of these difficulties Newton’s stature was such that the corpuscular hypothesis remained dominant for decades.
With the considerable benefit of hindsight there was actually no conflict because light has particle and wavelike aspects, both manifestations of a deeper reality. All particles show wave behaviour under appropriate conditions, for example electron and neutron beam diffraction are routine tools to probe molecular and crystal structures.
Tom’s image neatly illustrates light’s duality. The sharp shadows making the Brocken Spectre are fully explainable by assuming streams of photons travelling in straight lines. The glory can only be explained – and even then with much difficulty! – by invoking wave behaviour. The differences arise from the very considerable differences in scale. The fog droplets producing the glory are not much larger than the wavelength of the light and diffraction rules the day. The figure is a million times larger and whilst diffraction still occurs it requires a special look to find it.
|A view towards the bright lamps. The fog has produced a corona around each. The corona and glory are part of the same light scattering and diffraction process.|
|Here the image levels are adjusted to reveal the residual glory ring colours. A high pressure sodium lamp was probably the light source. While yellow-orange light predominates, pressure broadening gives the lamp a background wideband continuum emission.|