A balloon filled with a gas that has a different sound speed than that of air has been used as an
acoustic lens. One purpose of the lens is to show refraction of sound waves in an analogy to
geometric optics. We discuss the physics of the balloon lens demonstration. To determine the
validity of a gas-filled balloon as a classroom demonstration of an acoustic lens and to understand
the corresponding phenomena, its physics is considered analytically, numerically, and
experimentally. Our results show that although a geometric analogy is a good first-order
approximation, scattering theory is required to fully understand the observed phenomena. Thus this
demonstration can be adapted to a wide range of students, from those learning the basic principles
of refraction to advanced students studying scattering
