Diffraction is the phenomenon in which waves encounter an obstruction such as an aperture and are thereby redirected in new directions according to a principle first described by Huyghens over 300 years ago. While this project is concerned with visible light waves, the same principles apply to other types of waves, including sound and water waves. Interference between these diffracted waves results in distinctive patterns, called diffraction patterns. (If there is more than one aperture the patterns are generally called "interference patterns," but regardless of the number of diffracting objects both diffraction and interference are involved in forming the patterns.) Relatively close to the obstruction or aperture (in the "near field") the patterns vary with distance to the observing screen, but at large distances (in the "far field") the patterns remain the same except for an overall increase in size in proportion to distance. The transition from near-field to far-field can be quantified with the Fresnel number F = a ² / λ L, where a is the radius of the aperture, λ is the wavelength, and L is the distance between the aperture and the image plane. The near-field has Fresnel number roughly equal to or greater than 1, while the far-field has F << 1. Diffraction is often demonstrated with laser light incident on small apertures (eg 100 microns diameter). In such experiments the far-field pattern is easily observed by eye at a distance of several meters, but the rich complexity of the near-field patterns isn't evident as these occur just a few mm from the aperture on a microscopic scale.

The goal of our project is to explore these fundamental ideas with simple experiments in which coherent light from a red HeNe laser (wavelength 633 nm) is incident on a variety of pinhole apertures, using various techniques to record the resulting diffracton patterns at a range of distances from the near field into the far field. Our best results so far have been obtained with a 0.50 mm diameter commercial aperture - with this aperture the near-field transition (F=1) occurs at about 100 mm from the aperture. We magnified the near-field patterns with a 10x microscope objective and recorded them with a normal consumer camera. A distinctive feature of the patterns is the appearance of a dark central spot at certain distances L; at other distances a bright central spot is surrounded by one or more dark rings.