  We investigate a simple forced harmonic oscillator with a natural frequency
varying with time. It is shown that the time evolution of such a system can be
written in a simplified form with Fresnel integrals, as long as the variation
of the natural frequency is sufficiently slow compared to the time period of
oscillation. Thanks to such a simple formulation, we found, for the first time,
that a forced harmonic oscillator with a slowly-varying natural frequency is
essentially equivalent to diffraction of light.
