The optical properties of air and pure water (like rain) are well understood, and raindrops are nearly always spherical in shape.
With a little math (okay, I won't cover it here), it can be shown that the angle between incoming white sunlight and the outgoing rainbow colors is about 42 degrees.
To be more precise, the red light comes out at 42 degrees and blue light comes out at 40 degrees.
This is true anywhere the sunlight hits raindrops, not just where we see the colorful arc.
This fact is what enables us to predict exactly where the rainbow will be located, should one appear.
All we need to know is the direction, or axis, of the incoming sunlight.
With the sun directly at our back, the axis of sunlight is the line between our eye and the shadow of our head.
The shadow of our head is known as the anti-solar (opposite the sun) point.
Key to Locating Rainbows: the Angles
Point to Remember
The direction of the sun and the position of our eye (camera) are the only things that determine where a rainbow can form (not "will" form!).
Our rainbow can now be located at an angle 42 degrees off the axis of the sunlight.
You can think of this shape as a cone.
The centerline of the cone lines up with the sun, your eye, and the shadow of your head.
Imagine the cone so large it reaches all the way to the clouds, and where the touch is where the rainbow can appear.
Of course, where the cone enters the ground there can't be a rainbow unless you're high above looking down on the rain (or waterfall spray).
photo by Annie Boomer
photo by Annie Boomer
Annie Boomer photographed this lovely sunset (LEFT) on Alaska Bay (off Lake Huron), Pointe Aux Barques, MI (September 5, 2011).
That might absorb all of a photographer's attention, but a good rule is to keep looking behind you to see what else is happening opposite the sunrise or sunset.
In this case, Annie captured a rare and beautiful sight (RIGHT): this vertical rainbow just at sunset.
Wouldn't we all love the opportunity to witness such an awesome display, and maybe have our cameras ready!
Thanks, Annie, for sharing this incredible scene.
Sunset rainbows are special for three reasons.
(1) The sun's rays are nearly horizontal, so the top of the rainbow will be high in the sky.
In fact, a sunset rainbow is the widest arc you'll ever see from the ground: almost half of the full-circle rainbow can become visible, and you'll need a wide angle lens to capture it all.
This means the ends of the arc are nearly vertical as they intersect the horizon.
Sometimes only the end segment of the rainbow appears, and if you see a photo of a vertical rainbow at the horizon, you'll know it was made at sunset (or sunrise).
With a little geometry work and a sun angle chart, you can tell time using a rainbow at the horizon.
Ireland Sunset Rainbow near The Cliffs of Moher (photo by Shannon Field)
(2) At sunset, the sunlight contains more red hues and less blue (because of atmospheric scattering).
This will affect the appearance of the rainbow by emphasizing the red bands and muting the blue bands.
The same red tint will apply to anything on the ground illuminated by the sunset.
The effect can be quite dramatic.
Digital cameras can be fooled into over-correcting the red tint, so watch your color temperature setting (you don't want the red light rendered white).
(3) If you're lucky, you can get a rainbow against sunset clouds.
This phenomenon is gorgeous to behold, but photos seem to be few.
Be sure to turn around next time you're photographing a sunset, and see if the sky behind holds anything interesting.