Impact of shock power on spectral properties. Left: time evolution of the spectral properties in the UV and in the optical for the SN II model with an interaction power of 10⁴¹ erg s⁻¹ (black) and the model without interaction power (blue). Both models have, however, the same ejecta density structure (short-dashed curve in Fig. 1). For each epoch, the spectra were divided by the maximum flux of the brighter model and then stacked for visibility. Right: same as on the left, but now showing the spectra versus Doppler velocity and centered on the rest wavelength of Hα. The red curve shows the flux difference between the two models.