  We present an analytic model for bolometric light curves which are powered by
the interaction between supernova ejecta and a dense circumstellar medium. This
model is aimed at modeling Type IIn supernovae to determine the properties of
their supernova ejecta and circumstellar medium. Our model is not restricted to
the case of steady mass loss and can be applied broadly. We only consider the
case in which the optical depth of the unshocked circumstellar medium is not
high enough to affect the light curves. We derive the luminosity evolution
based on an analytic solution for the evolution of a dense shell created by the
interaction. We compare our model bolometric light curves to observed
bolometric light curves of three Type IIn supernovae (2005ip, 2006jd, 2010jl)
and show that our model can constrain their supernova ejecta and circumstellar
medium properties. Our analytic model is supported by numerical light curves
from the same initial conditions.
