The size of the light-emitting region is a key parameter toward understanding the light-emitting processes in a sonoluminescing bubble. Here we present measurements of interference effects from particles with a diameter of approximately $2\mu \mathrm{m}$ situated $6–10\mu \mathrm{m}$ from a sonoluminescing bubble. From the angular size of the pattern and from an estimated distance to the particles we conclude that the light-emitting region of a sonoluminescing bubble is smaller than commonly believed $\{$see, e.g., S. Hilgenfeldt, S. Grossman, and D. Lohse [Nature (London) 398, 402 (1999)]$\}$. We argue that an upper limit of the size of the light-emitting region is approximately 200 nm.

• Received 26 May 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.144301