Why Do Rainbows Form?

Conditions have to be just right for a rainbow to form in the sky. With the correct amount of moisture after a rain and the perfect angle of the sun, rainbows will be clearly visible to someone standing on the earth. Some may even be crisp enough for the viewer to see distinct color separations across the entire spectrum.

One explanation from a science text states that rainbows form from a specific trail of events/conditions, including refraction, dispersion and reflection. In basic terms, this explanation states that these separate phenomena and the rainbow itself happen when sunlight, air and moisture interact in a certain way.

In general terms, the steps include sunlight striking a drop of rain, reflection of some light, refraction of another portion of light and the “splitting” of sunlight into the colors of the spectrum. Light exits the individual rain drops and goes through similar processes “on the other side” of the moisture.

In addition to the absolute necessity of raindrops in the air, the sunlight must hit those drops of moisture at just the right angle. The color dispersion is best when the sun is low, just after rising or just before setting. We may see some of the sunlight is reflected or turned back, similar to what happens when we see a reflection in a stream or lake. If you could stop a raindrop and look into it from very close you would be able see some reflection of your face.

Sunlight that isn’t reflected is refraction. This process involves bending of light rays that normally travel in a straight line. Since the water drop is much denser than the air around it the light hits the surface of the drop and bends. We see sunlight as being white or clear but it contains a spectrum of different colors. These colors are determined by the individual wavelengths and speed of travel for each color.
When white/clear sunlight strikes a raindrop the speeds of the different colors move in slightly different ways. Basically, the colors separate, allowing us to see them as a rainbow. But for a rainbow to be clearly visible, the sunlight must experience some internal reflection and speed up as it exits the raindrop. Light is dispersed upon exiting the drop of rain.

The color separation results in a spectrum of red, orange, yellow, green, blue, indigo and violet. This same “rainbow” effect can be seen through the mist of a garden hose or fountain as well, if the light comes at just the right angle. The best rainbow views come when part of the sky is still dark with clouds. This gives the light and the colors more contrast. Scientists have even determined that the best rainbows are produced when the sunlight is coming from an angle of about 40 degrees. This means the sun has to be relatively low in the morning, after dawn, or in the evening, just before sunset. It’s possible to see a secondary rainbow if the sun angle is perfect.