Number 209 (Story #1), January 8, 1995 by Phillip F. Schewe and Ben Stein THE INDEX OF REFRACTION FOR SODIUM ATOM WAVES through various gases has been measured, providing new details about interatomic forces between sodium and the individual gases. Under certain experimental conditions, whole atoms, like light, can behave as rippling waves rather than pointlike particles. In the past few years, researchers have successfully demonstrated some of atoms' hard-to-detect wavelike properties using atom interferometers, devices that split up atom waves and recombine them to form interference patterns. Now, David Pritchard of MIT (617-253-6812) and his co-workers have passed one portion of a sodium atom wave through one of numerous gas samples (such as helium, neon, argon, ammonia, and water) and then recombined it with another portion of the wave that did not pass through the gas. From the resulting interference pattern the researchers derived the index of refraction, basically a measure of the "bending" angle of the sodium atom ray as it traverses the gas medium. From the index of refraction researchers extracted new details about the long-range forces between the sodium atom and the gases. Of the gases studied, they have found that helium behaves most like a hard sphere in its interactions with sodium, exerting the weakest long-range attraction, while xenon has the most dominant long-range attraction. (J. Schmiedmayer et al., 13 Feb. in Physical Review Letters.)