Rainbows, with their vibrant colors arcing across the sky, are a natural spectacle often seen after rain. This common association leads many to believe rain is an absolute requirement. However, rainbows can form without rain, inviting a closer look at the conditions that allow them to grace our skies.
How Rainbows Form
A rainbow is an optical phenomenon resulting from the interaction of sunlight with water droplets. The process begins when white sunlight enters a spherical water droplet, which acts like a miniature prism. As light passes from air into denser water, it slows down and bends, a process called refraction. This refraction separates the white light into its component colors because each color bends at a slightly different angle.
After entering the droplet and refracting, the light travels to the back inner surface. Here, it undergoes internal reflection, bouncing off the inside wall. The light then travels back to the front of the droplet and refracts once more as it exits the water and re-enters the air. This sequence of two refractions and one internal reflection causes the sunlight to spread into a continuous spectrum of colors, from red to violet.
The spherical shape of water droplets is crucial for this light manipulation. Each color emerges from the droplet at a specific angle relative to the incoming sunlight. This precise bending and splitting of light within countless individual water droplets combine to create the colorful arc visible to an observer.
Water Droplets Beyond Rain
Rainbows do not exclusively rely on falling rain; they simply require airborne water droplets illuminated by sunlight. Various natural and artificial sources can provide these droplets, allowing rainbows to form in unexpected places.
Mist and fog, for instance, consist of numerous tiny water droplets suspended in the air. When sunlight shines through these atmospheric conditions, it can create phenomena known as fogbows or mistbows. These often appear as a white or pale arc, sometimes with faint colors, due to the smaller size of the water droplets which causes light to diffract differently.
Waterfalls and ocean spray are other common sites for non-rain rainbows. The forceful movement of water generates a fine spray of airborne droplets. Similarly, artificial sources such as garden sprinklers or hoses can create a sufficient mist to produce a localized rainbow effect.
Even dew on grass can form a “dewbow,” particularly in the early morning or late evening when the sun is low. These occur when sunlight refracts and reflects off the dew drops clinging to surfaces, creating a circular display of colors on the ground.
Optimal Viewing Conditions
Seeing a rainbow requires more than just water droplets and sunlight; the observer’s position relative to both the sun and the droplets is crucial. For a primary rainbow to be visible, the sun must always be behind the observer. The water droplets must then be positioned in front of the observer, allowing the sunlight to pass through them and reflect back towards the eye.
The light that forms a rainbow is concentrated at a specific angle, approximately 40 to 42 degrees, relative to the line extending from the sun through the observer’s eye to their shadow. If the sun is too high, typically above 42 degrees in the sky, the rainbow will appear below the horizon and will not be visible from the ground.
While rainbows are optically full circles, they are almost always seen as semicircular arcs from the ground. This is because the Earth’s surface obstructs the lower portion of the circle. To observe a full circular rainbow, one would need to be at a higher elevation, such as in an airplane, where there are water droplets below the observer’s horizon.