ADOLFO MUNOZ

In this paper we derive a physically based model for simulating rainbows. Previous techniques for simulating rainbows have used either geometric op-tics (ray tracing) or Lorenz-Mie theory. Lorenz-Mie theory is by far the most accurate technique as it takes into account optical effects such as dis-persion, polarization, interference, and diffraction. These effects are critical for simulating rainbows accurately. However, as Lorenz-Mie theory is re-stricted to scattering by spherical particles, it cannot be applied to real rain-drops which are non-spherical, especially for larger raindrops. We present the first comprehensive technique for simulating the interaction of a wave-front of light with a physically-based water drop shape. Our technique is based on ray tracing extended to account for dispersion, polarization, inter-ference, and diffraction. Our model matches Lorenz-Mie theory for spheri-cal particles, but it also enables the accurate simulation of non-spherical par-ticles. It can simulate many different rainbow phenomena including double rainbows and supernumerary bows. We show how the non-spherical rain-drops influence the shape of the rainbows, and we provide a simulation of the rare twinned rainbow, which is believed to be caused by non-spherical water drops