Sensitivity of wave-optics diffraction to the external potential. Each panel shows the caustics associated to the microlens, the time-domain amplification I(τ), and the corresponding frequency-domain amplification factor ∣F(f)∣, considering the total and diffractive contributions (labeled “WO”). Top panels: varying the external potential strength, assuming κ = γ, at fixed microlens position. Bottom panels: varying the microlens position angle at fixed external potential. Although macroscopic magnification is achromatic in geometric optics, the external potential modifies the microlensing caustics and time-delay response, producing frequency-dependent distortions in F(f).