This paper describes the formation of a circumzenithal arc for the purpose of teaching light and
optics. A circumzenithal arc, an optic formation rarely witnessed by people, is formed in this study
using a water-filled cylindrical glass illuminated by sunlight. Sunlight refracted at the top and
side surfaces of the glass of water is dispersed into its constituent colours. First, multi-colour
arcs are observed on paper at the bottom of the glass. Then, a single arc for each colour is
observed on the floor when the rays are allowed to propagate to the furthest points from the glass.
The change in observations is explained by formulating an equation for the geometry of the
situation. The formula relates each point on the first refracting surface for an incoming light ray
to a point further from the second refracting surface. Then, a parallel graph is drawn to visualize
the superposition of colours to the formation of a single arc. The geometrical optics studies in
this paper exempl…