A rainbow is a spectacular display of light and color that appears when sunlight interacts with tiny water droplets suspended in the atmosphere. It is often mistakenly viewed as a fixed, static object in the distance. However, the rainbow’s apparent position constantly changes relative to the viewer because it is an optical event anchored to the observer’s precise location, not a physical object.
The Fixed Geometry of Rainbow Formation
The appearance of a primary rainbow is governed by a precise geometric rule. For a rainbow to be visible, the sun must be positioned directly behind the observer, shining toward an area containing moisture, such as rain or mist. This setup ensures that the light enters the water droplets from the correct angle for the phenomenon to occur.
When sunlight enters a spherical raindrop, it first refracts, separating the white light into its constituent colors. This light then reflects off the inner back surface of the droplet before refracting again as it exits toward the viewer’s eye. This double process of refraction and single internal reflection creates the colored light seen in the bow.
The most intense light returning to the observer always emerges at a fixed angle of approximately 42 degrees from the incoming sunlight’s path. This angle means every visible rainbow is a segment of a cone of light with the observer’s eye at its apex. The center point of this cone is always directly opposite the sun, a spot known as the anti-solar point.
Movement is Relative to the Observer
The rainbow appears to move when the observer moves because of the fixed 42-degree geometry. Since the rainbow is defined by the angle between the sun, the water droplet, and the eye, changing position requires a new set of water droplets to satisfy this angular requirement. The rainbow seen is not a single, distant arc but a continuous stream of light paths from an ever-changing collection of raindrops.
For example, when driving toward a visible rainbow, the arc appears to shift forward at the same rate as the vehicle. This happens because the observer’s cone of vision is constantly directed toward new, previously unseen droplets to maintain the 42-degree angle relative to the stationary sun. The light rays reaching the eye are essentially parallel, making the source of the rainbow appear infinitely distant. Therefore, its position does not seem to change as one approaches.
Even without the observer moving, the rainbow can appear to shift over a longer period due to the Earth’s rotation. As the sun moves across the sky, the anti-solar point—the center of the rainbow’s circle—also moves, causing the entire arc to rise or fall relative to the horizon. This apparent movement is simply the fixed geometric relationship adjusting to the changing position of the light source.
Why the Rainbow Cannot Be Chased
Because the rainbow is an optical phenomenon defined by the observer’s perspective, it is impossible to reach a single, fixed location where the rainbow ends. The arc is not a target painted onto the sky; it is an effect that exists only from the viewpoint of the person seeing it. Every individual sees a slightly different rainbow, composed of light reflected from a unique set of water droplets.
If two people stand side-by-side, the light forming the rainbow in one person’s eye comes from different water droplets than the light forming the rainbow in the other person’s eye. If an observer attempts to walk toward the spot where the rainbow meets the ground, the 42-degree cone of vision moves with them, causing the arc to perpetually retreat.
The rainbow has no physical existence in a specific location within the atmosphere; it is merely the collective light from droplets positioned correctly relative to the sun and the observer’s eye. This explains why the full, 360-degree circle of a rainbow is only visible from an elevated position, such as a high mountain or an airplane, where the ground does not obstruct the lower portion of the light cone.