A rainbow, an atmospheric display, often appears as a vibrant arc stretching across the sky. Many believe its natural form is a semi-circular band of color. This prompts a question about its true geometric nature. Is a rainbow just an arc, or does its shape extend beyond what is typically observed from the ground? Exploring the science behind how rainbows form reveals their complete shape, challenging common perceptions.
The Rainbow’s True Form
A rainbow is a full circle, not merely an arc. This complete circular shape results from sunlight interacting with water droplets in the atmosphere. When sunlight enters a spherical raindrop, it refracts, then reflects off the inner surface of the droplet, and finally refracts again as it exits. This process disperses the white sunlight into its component colors, much like a prism.
Each water droplet acts as a tiny prism, scattering light at specific angles. Red light exits the droplet at approximately 42 degrees relative to the incoming sunlight, while violet light exits at about 40 degrees. For an observer, raindrops reflecting light at these precise angles form a cone of light with the observer’s eye at its tip. Millions of such droplets, each contributing one specific color to the observer’s eye, collectively create the perception of a circular spectrum.
Why an Arc Appears
Despite its true circular nature, a rainbow almost always appears as an arc when viewed from the ground. This occurs because the Earth’s surface obstructs the lower portion of the full circular light cone. The horizon blocks the view of water droplets that would form the bottom part of the circle, making the rainbow appear to end at the ground.
The observer’s position relative to the sun is crucial; the center of the rainbow always lies in the direction directly opposite the sun. When the sun is higher in the sky, more of the circular rainbow falls below the horizon, resulting in a smaller visible arc. Conversely, as the sun approaches the horizon, a larger portion of the circular rainbow becomes visible, potentially appearing as a full semicircle at sunrise or sunset.
How to See a Complete Circle
Observing a full circular rainbow requires specific conditions to overcome ground obstruction. One common way to witness this is from an elevated vantage point, such as an airplane. From an aircraft, the observer is high enough to see water droplets below the horizon, allowing the entire circular light cone to be visible without interference.
Another opportunity to see a complete rainbow circle is near powerful waterfalls or in areas with significant mist, like a garden hose spray. By standing with the sun behind you and spraying water, or by observing the mist from a large waterfall, you can create or see the conditions for the full circular display. These scenarios provide a continuous field of water droplets both above and below eye level, allowing the full 360-degree rainbow to form.