Experimental EELS spectra recorded at grazing incidence on C nanotubes with different numbers of layers and associated r/R ratios (bottom curves) and simulated ones (top curves). The split peak at 15 eV corresponds to a tangential mode with field lines parallel to the surfaces of the nanotube and at 19 eV to a radial mode with field lines perpendicular to the surfaces of the nanotube. One notices the vanishing of the radial mode for the single-wall nanotubes (from Stéphan et al. [108]).