Optical transmission spectrum of GJ 3470b. The GTC/OSIRIS measurements and re-analyzed LBT/LBC measurements are shown in navy-blue circles and purple squares with error bars, respectively. The dotted line shows the best-fitting slope of the GTC/OSIRIS measurements. The dashed line shows the best-fitting slope of the combined GTC/OSIRIS and LBT/LBC measurements. The solid line shows a Rayleigh scattering slope based on the physical parameters of GJ 3470b with a mean molecular weight of μ_m = 2.37 and a Bond albedo of A_B = 0.3.