Teaching Physics with the Physics Suite

Edward F. Redish

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Modern theories of atoms describe them not just as little particles but also say they should have wave properties. If this is so, then passing a beam of atoms through a double slit should produce an interference pattern. The figure at the right, taken from an article in Physical Review Letters,* shows the result of passing a beam of Helium atoms through a double slit.

The slits are 8 μm apart and the screen collecting the atoms is 64 cm away from the slits. If the peaks that are seen in the figure are two-slit interference peaks, what would be the wavelength of the atoms? Explain your reasoning. (Note: This was the first experiment demonstrating interference with whole atoms. The data is a bit ragged since it was difficult to get a sharp wavelength – which is determined by the velocity of the atoms in the beam. The agreement of this measured value with the predicted theoretical wavelength was a convincing argument that the experimenters had actually seen the phenomenon.)