Most people understand that a rainbow is light spread into various colors by airborne water drops. Though a rainbow can feel like a random, unpredictable phenomenon, the natural laws governing rainbow are actually quite specific and predictable, and understanding these laws can help photographers anticipate a rainbow and enhance its capture.
The sun’s visible wavelengths are captured by our eyes and interpreted by our brain. When our eyes take in light comprised of the full range of visible wavelengths, we perceive it as white (colorless) light. Color registers when some wavelengths are more prevalent than others. For example, when light strikes an opaque (solid) object such as a tree or rock, some of its wavelengths are absorbed; the wavelengths not absorbed are scattered (reflected). Our eyes capture this scattered light, send the information to our brains, which interprets it as a color. When light strikes water, some is absorbed, some passes through to reveal the submerged world, and some light is reflected by the surface as a reflection.
To understand the interaction of water and light that creates a rainbow, it’s simplest to visualize what happens when sunlight strikes a single drop. Light entering a water drop slows and bends, with the shorter wavelengths bending more than the longer wavelengths: refraction. Refraction separates the originally homogeneous white light into the myriad colors of the spectrum: red, orange, yellow, green, blue, indigo, violet (in that order).
But simply separating the light into its component colors isn’t enough to create a rainbow. Actually seeing the rainbow spectrum caused by refracted light requires that the refracted light be reflected back to our eyes somehow.
A raindrop isn’t flat like a sheet of paper, it’s spherical, like a ball. Light that was refracted when it entered the front of the raindrop, continues through to the back of the raindrop, where some is reflected. To view a rainbow, our eyes must be in the correct position to catch this reflected spectrum of color—fortunately, this angle is very consistent and predictable.
Red light reflects at 42 degrees, violet light reflects at 40 degrees, while the other spectral colors reflect back between 42 and 40 degrees. That’s why the top color of the primary rainbow is always red, the longest visible wavelength; the bottom color is always violet, the shortest visible wavelength.
Every raindrop struck by sunlight creates a rainbow somewhere. But just as the reflection of a mountain peak on the surface of a lake is visible only when viewed from the angle the reflection bounces off the lake’s surface, a rainbow is visible only when you’re aligned with the 42 – 40 degree angle at which the raindrop reflects light’s refracted spectrum of rainbow colors.
Lucky for most of us, viewing a rainbow requires no knowledge of advanced geometry. To locate or anticipate a rainbow, put your back to the sun and picture an imaginary line originating at the sun, entering the back of your head, exiting between your eyes, and continuing into the landscape in front of you—this line points to the “anti-solar point,” an imaginary point exactly opposite the sun from your viewing position.
It helps to remember that your shadow always points toward the anti-solar point—and toward the center of the rainbow, which forms a 42 degree circle around the line connecting the sun and the anti-solar point. Unless we’re in an airplane or atop a mountain peak, we don’t usually see the entire circle because the horizon gets in the way. So when you find yourself in a mixture sunlight and rain, locating a rainbow is as simple as following your shadow and looking skyward—if there’s no rainbow, the sun’s probably too high.
Sometimes a rainbow appears as a majestic half-circle, arcing high above the distant terrain; other times it’s merely a small arc hugging the horizon. As with the direction of the rainbow, there’s nothing mysterious about its varying height. Remember, every rainbow would form a full circle if the horizon didn’t get in the way, so the amount of the rainbow’s circle you see (and therefore its height) depends on where the rainbow’s arc intersects the horizon.
While the center of the rainbow is always in the direction of the anti-solar point, the height of the rainbow is determined by the height of the anti-solar point, which will always be exactly the same number of degrees below the horizon as the sun is above the horizon. It helps to imagine the line connecting the sun and the anti-solar point as a fulcrum, with you as the pivot—picture yourself in the center of a teeter-totter: as one seat rises above you, the other drops below you. That means the lower the sun, the more of the rainbow’s circle you see and the higher it appears above the horizon; conversely, the higher the sun, the less of the rainbow’s circle is above the horizon and the flatter (and lower) the rainbow appears.
Assuming a flat, unobstructed scene (such as the ocean), when the sun is on the horizon, so is the anti-solar point (in the opposite direction), and half of the rainbow’s 360 degree circumference will be visible. But as the sun rises, the anti-solar point drops—when the sun is more than 42 degrees above the horizon, the anti-solar point is more than 42 degrees below the horizon, and the only way you’ll see a rainbow is from a perspective above the surrounding landscape (such as on a mountaintop or on a canyon rim).
Of course landscapes are rarely flat. Viewing a scene from above, such as from atop Mauna Kea in Hawaii or from the rim of the Grand Canyon, can reveal more than half of the rainbow’s circle. From an airplane, with the sun directly above you, all of the rainbow’s circle can be seen, with the plane’s shadow in the middle.
Not all of the light careening about a raindrop goes into forming the primary rainbow. Some of the light slips out the back of the raindrop to illuminate the sky, and some is reflected inside the raindrop a second time. The refracted light that reflects a second time before exiting creates a secondary, fainter rainbow skewed 50 degrees from the anti-solar point. Since this is a reflection of a reflection, the colors of the secondary rainbow are reversed from the primary rainbow.
And if the sky between the primary and secondary rainbows appears darker than the surrounding sky, you’ve found “Alexander’s band.” It’s caused by all the light machinations I just described—instead of all the sunlight simply passing through the raindrops to illuminate the sky, some of the light was intercepted, refracted, and reflected by the raindrops to form our two rainbows, leaving less light for the sky between the rainbows.
Understanding the optics of a rainbow has practical applications for photographers. Not only does it help you anticipate a rainbow before it happens, it also enables you to find rainbows in waterfalls.
A rainbow caused by sunlight on rain can feel random because it’s difficult to know exactly where the rain will fall, when the sun will break through, and exactly where to position yourself to capture the incongruous convergence of rainfall and sunshine. A waterfall rainbow, on the other hand, can be predicted with clock-like precision because we know exactly where the waterfall and sun are at any give time—as long as clouds don’t get in the way, the waterfall rainbow appears with clock-like precision.
Yosemite is my location of choice for waterfall rainbows, but maybe there’s a waterfall or two near you that might deliver. Just figure out when the waterfall gets direct sunlight early or late in the day, then put yourself somewhere on the line connecting the sun and the waterfall. And if you have an elevated vantage point, you’ll find that the sun doesn’t even need to be that low in the sky.
Spring in Yosemite is waterfall rainbow season, and I know exactly where to be and when to be there for both of Yosemite Valley’s major waterfalls. In fact, given the variety of vantage points for viewing each of these falls, I can usually get two or three rainbows on each fall on any given day.
In addition to clouds, there are other variables to deal with. One is the date, because the path and timing of the sun’s arc across the sky changes with each passing week. Another thing that can throw the timing off slightly is the amount of water in the fall—following a wet winter the spring runoff increases, and with it the amount of mist. Generally, the more mist, the sooner the rainbow will appear and the longer it lasts. And finally there’s wind, which spreads the mist and usually improves the rainbow by increasing its size.
While all these variables make it difficult for me share the exact schedule of Yosemite’s waterfall rainbows from the variety of vantage points, I can give you some general guidance: look for a rainbow on Yosemite Falls in the morning, and Bridalveil Fall in the afternoon. And if you don’t mind a short but steep hike, you can also find a rainbow on Vernal Fall in the afternoon.
Understanding rainbow optics can even help you locate rainbows that aren’t visible to the naked eye. A “moonbow” (lunar rainbow) is a rarely witnessed and breathtaking phenomenon that follows all the natural rules of a daylight rainbow. But instead of resulting from direct sunlight, a moonbow is caused by sunlight reflected by the moon.
Moonlight isn’t bright enough to fully engage the cones in your eyes that reveal color, though in bright moonlight you can see the moonbow as an arcing monochrome band. But a camera on a sturdy tripod can use its virtually unlimited shutter duration to accumulate enough light to bring out a moonbow in full living color. Armed with this knowledge, all you need to do is put yourself in the right location at the right time.
Probably the best known moonbow is the one that appears at the base of Lower Yosemite Fall each spring. Usually viewed from the bridge at the base of Lower Yosemite Fall, the best months are April, May, and June, with May probably being the best combination of moonlight angle and ample water. Unfortunately, this phenomenon isn’t a secret, and the bridge at the base of Lower Yosemite Fall can be quite crowded on spring full moon nights. And it high runoff springs, it can also be extremely wet (pack your rain gear). The base of Upper Yosemite Fall can also have a moonbow when viewed from the south side of Cook’s Meadow, especially in wet springs.
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