Rainbows are actually full circles, but the ground usually blocks the bottom half from view.

A rainbow is an optical phenomenon that occurs at a specific angle of 42 degrees from the anti-solar point, which is the point directly opposite the sun from the observer's perspective. When sunlight enters a spherical raindrop, it refracts, reflects off the back of the drop, and refracts again as it exits. This process disperses the light into its component colors, creating a circular cone of light.The reason we usually see a semi-circular arc is due to the geometry of the Earth's horizon. For an observer on the ground, the lower half of the 42-degree cone is physically blocked by the planet itself. This means the water droplets necessary to complete the circle are located below the horizon where sunlight cannot reach them to reflect back to your eyes.To witness a full-circle rainbow, often called a 'glory' or 'circular rainbow,' the observer must be at a high altitude with water droplets both above and below their line of sight. Pilots and passengers in aircraft frequently report these sightings when flying between the sun and a cloud layer. In 2014, a drone captured a rare and famous video of a perfectly circular rainbow over Perth, Australia, providing clear visual evidence of this geometry.Sir Isaac Newton first mathematically described the physics of the rainbow in his 1704 book 'Opticks.' He explained how the refractive index of water causes different wavelengths of light to bend at slightly different angles. This scientific foundation confirms that the circular shape is a constant property of light physics, limited only by the observer's physical environment.