In the shadow image of a spherical gas bubble, high intensity rings are visible, i.e., glare circles. These can be used to obtain a more precise estimate of the bubble diameter than can be obtained from the shadow contour. The glare circle diameter can also be used to determine the relative refractive index by comparing it with the shadow diameter. The precision of this refractive index measurement reaches the third decimal, which is demonstrated experimentally. Thus, one can simultaneously determine the bubble diameter (from the shadow contour) and the relative refractive index (from the glare circle).
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