Supporting Information We can solve for the average refractive index of the polystyrene particle film layer using the Maxwell-Garnett Effective Medium Approximation: eff m 2(1 i ) m (1 2 i ) i (2 i ) m (1 i ) i (Equation 1) where e m and e i are the dielectric constants for polystyrene and air respectively and f i represents the filling fraction of the air. By using the square of the refractive index as the dielectric constant, we have e m = 2.56, e i = 1, f i = 0.3, we find eff = 2.01. Taking the square root, we find neff = 1.42 ---------------------------------------------------------------------------------------------------------- For normal incidence, the Fresnel reflection between two interfaces is given by the following formula: n1 n 2 2 R n1 n 2 (Equation 2) We solve for R in order to obtain a value for the reflected intensity between air and a layer of polystyrene particles. Using n1 = 1.42 (polystyrene with air inclusions) and n2 = 1 (air), R = 0.03 corresponding to 3% reflectance. This corresponds to specular reflection. The error in the measurement from the spectrophotometer can be attributed to background diffuse reflection.