Rainbows have always been carrying with them an aura of legend and mystery. The ancient Indian tribes in North America have the belief that a rainbow was the bridge of death and life. But the Romans see it as positive sign for their sun gold.
In reality, nothing is mystical, mythical or mysterious about rainbows. Most people considered that the multi-colored arcs which appear in the eastern horizon after rainstorm is a breathtaking and beautiful gift of nature. But the creation of rainbow does follow several laws of physics that Descartes, a scientist that started seriously investigating it in the 17th century.
The formula is involving variables such as reflection and refraction of light, color spectrums, prisms, antisolar points and many other complicating sounding words. But the truth is that the rainbow formation is a simple science for so long as all of the factors are present the moment it is created.
Light is composed of color series, ROYGBIV or red, orange, yellow, green, blue, indigo and violet needless to say other light such as ultraviolet that is not as detected easily by the human eye. Light has varying wavelengths as well. Wavelengths that are the longest are visible to the human eye as the red color while the shortest wavelength the violet color with short list of other colors between them. When the light is passing through anything it bends or become refracted light. No two objects have similar refractive index.
When the light is passing through an object such as ornamental crystals that most people hang in their windows, every color created bends according the corresponding wavelength and spread in order for the colors of the window to be formed on the opposite wall.
The same thing would occur when the sun is peeking out from behind the clouds during or after the rainstorm. Raindrops should be present still in the atmosphere to form a rainbow. Thunderstorms and rain shows are not that only times a rainbow can appear. They can also be observed in the bank of mist in the spray billowing from waterfalls or made by lawn sprinklers or any situation where light is reflecting through water or objects such as crystal or glass.