The cubic-to-rhombohedral phase transition in the superionic conductor RbAg4I5 is characterized by strong birefringence in the rhombohedral phase. Precursive fluctuations in the cubic phase lead to complete depolarization of transmitted light at Tc=208 K. We show that the depolarization increases exponentially as a power of reduced temperature. For the case of a linear relationship between the order parameter and the birefringence, as in RbAg4I5, the power of temperature in the exponential is the critical exponent γ which governs the susceptibility-like response function. An experimental value γ=1.25±0.1 is obtained which agrees well with Ising-like values, and scales with other critical exponents. We believe this to be the first observation of a critical depolarization effect.