As you might expect it takes rain to make a rainbow but sunshine and some geometry play an equally major role in making a rainbow. Rainbows are optical phenomenon that appear when sunlight and atmospheric conditions are just right, but to see them a viewer needs to be in the right spot as well. The main cause of a rainbow goes even deeper than just the right set up, it also involves refraction and reflection of sunlight as it interacts with raindrops falling through the atmosphere.
Sunlight is white light and is made up of all the wavelengths of the visible spectrum. Of course that includes the colors we see in a rainbow. Those rainbow colors in order from the outer arc to inner arc are Red, Orange, Yellow, Green, Blue, Indigo and Violet. The acronym R.O.Y.G.B.I.V. can help us remember these colors. Roy G Biv is a colorful character. This color scheme is reversed for the secondary rainbow in a double rainbow situation.
The colors we see in the rainbow are a result of sunlight hitting individual water drops where reflection and refraction of the light gives us a rainbow. When sunlight hits a water drop some of the light enters the drop. As the light moves through the drop it is slowed a bit which causes the light rays to bend. The water acts like a prism bending certain wavelengths of light more than others. Violet (the shortest wavelength of visible light) bends the most, red (the longest wavelength of visible light) bends the least. The sunlight then hits the back inside surface of the water drop and is reflected back toward the observer and out the front of the drop.
The now separated sunlight colors are visible in a 42° arc from the observer. This arc can be a full circle when observed from an elevated angle like from an airplane.
The sky inside of a rainbow is brighter and more well lit than the sky outside of the primary rainbow. That is because the light is also scattered in all directions from the interaction with raindrops. Inside the arc of a rainbow the scattered light overlaps and is more intense inside the circumference of the rainbow.
Many times we are also treated to a secondary rainbow as well, that is fainter and has a larger arc (51°) than the primary rainbow. This secondary rainbow is caused by the light hitting the raindrops at just the right angle so that two reflections are realized inside the individual drops. The secondary rainbow’s colors are reversed from the primary with red on the bottom and violet on the top. The secondary rainbow is wider (8°) than the primary rainbow as well.
To see a rainbow the sun needs to be behind the observer shining into water droplets in front of the observer. Rainbows are usually seen in the western sky in the morning and the eastern sky when the sun angle is just right. Generally you won’t see a rainbow from ground level unless the sun is lower than 42° above the horizon.
Rainbows can be caused by many forms of airborne water. These include not only rain, but also mist, spray, and airborne dew. The spray from a garden hose is a great place to look for rainbows in your own backyard.
For Weather Nation: Meteorologist Mike Morrison