Around the Rainbow

Rainbows appear circular because they are formed by light refracting, reflecting, and dispersing inside spherical raindrops at a fixed angle of about 42° relative to the observer’s line of sight, meaning the geometry is always circular around the antisolar point.

On Earth, we usually see arcs because the ground blocks the lower part of the circle, but from high vantage points (like planes or mountains) a full circle can be visible.

Since the shape is dictated by spherical droplets and uniform refraction, rainbows cannot naturally appear as squares, triangles, or other shapes under normal atmospheric conditions.

However, variations can occur: elliptical bows may form if raindrops are flattened by air resistance, and halos or arcs with different radii can appear if ice crystals or non-water particles scatter light differently.

Thus, while the “true” rainbow remains circular, other optical phenomena in different media can produce arcs of varying shapes.