  We studied optical absorption of single-walled carbon nanotubes by varying
the dielectric environment. For the two different components of the broad UV
absorption feature conventionally referred to as the pi-plasmon, we find that
the component at 5.0 - 5.3 eV exhibits remarkable spectral changes, based on
which we attribute this to a dipolar radial surface plasmon. However, the
component at ~4.5 eV remains unchanged, raising a fundamental question as to
its conventional attribution. We discuss its relation with the absorption
feature at ~4.5 eV in graphite arising from an interband transition.
