As should be apparent from the entries, comments and illustrations on this website, central to modern crystallography is the phenomenon of ‘diffraction’ of X-rays by crystals. It is the critical experimental observation in crystallography. Now, I would like to present a brief blog commentary to highlight the fact that the ‘diffraction’ effect is NOT only obtained from crystals and X-rays. It can be observed in other situations combining the interaction of ‘matter’ and ‘radiation’. I will provide some brief background and I will discuss a serendipitous -common day- observation that will make you look through windows in the future in a different way.
The experiment was prompted by the suggestion by Max von Laue (German physicist, 1879-1960) that perhaps, if the X-rays were waves, they could be diffracted by a piece of matter that was supposed to have a regular atomic structure. This hypothesis about the internal structure of crystals had been suggested before by A. Bravais (French physicist, 1811-1863) and Abbé R. J. Haüy (French mineralogist, 1743-1822). It was the remarkable achievement of the experimental skills of Laue’s collaborators W. Friedrich and P. Knipping, that allowed to prove both things at the same time: the wave nature of the X-rays and the internal regular structure of crystals (Figs. 1a,b).
All of this seems to be far away from our regular daily life routines. However, during a casual look at a parking lot in a condominium in Norwalk, CT, visiting family, I was struck by how obvious the phenomenon of diffraction was. Having my cell phone readily available, I was able to photograph and document the observation (see accompanying images, Figs. 2-4).
The first image (Fig. 2, left), shows the original photo from behind a balcony inside the condominium, with me taking the photo and four bright spots corresponding to the four strong lights in the parking lot. Careful observation of the bright images shows the central light spots coming from the lights and then something else (Fig.3) (right). Kind of a square cross pattern. Aha!!!.
In addition, near the central bright spot, one can clearly see alternating bright and dark short lines along the horizontal and vertical directions. What is this? Where are these additional features coming from? Any ideas?
Finally, in the last close-up images (Fig. 4a,b, below), one can see that there is a wire ‘screen-door’ in front of the glass through which I am seeing the image: look closely on the lower left-hand part of the two images.
Aha!!. The alternating dark-bright lines along the horizontal-vertical directions (i.e. the cross) are the ‘diffraction pattern’ of the bright lights in the parking lot caused by the presence of the ‘screen-door’. You can convince yourself of this because if you slide off the wire ‘screen-door’, all you see is the four lights. The alternating dark and bright bands provide information about the distances between the wires in the ‘screen-door’ and their directions (square mesh). Now you may capture the essence of modern crystallography: to unravel the structure of the ‘diffracting’ object from the ‘diffraction pattern’, without knowing what it is beforehand. In the experimental setting, a small circular ‘beam stop’ is placed in the center to ‘block’ the bright central spot and enhance the visibility of the diffraction pattern.
From this brief discussion and daily life example, it should be apparent that ‘diffraction’ is a very different phenomenon from the ones we are all more familiar with of ‘reflection’ by a mirror surface, and ‘refraction’ by a water surface. Diffraction occurs when the wavelength of the incoming radiation is ‘commensurate’ with the internal dimensions of the diffracting object: X-rays to crystals vs. light waves to wire ‘screen-doors’. What does ‘commensurate’ mean: approximate the same internal dimensions or distances. The wavy pattern of the radiation repeats within approximately the same distances as the internal repeats of the interfering object.
I will conclude with a well-known quotation from Louis Pasteur (1822-1895): ‘Chance only favors the prepared mind’.
In the future, please, be alert and prepared to observe the unexpected. It will enrich your life and you may discover something new.