My relationship with Yosemite rainbows goes all the way back to my childhood, when a rainbow arcing across the face of Half Dome made my father more excited than I believed possible for an adult. I look back on that experience as the foundation of my interest in photography, my relationship with Yosemite, and my love for rainbows. So, needless to say, photographing a rainbow in Yosemite is a pretty big deal for me.
A few years ago the promise (hope) of lightning drove me to Yosemite to wait in the rain on a warm July afternoon. But after sitting for hours on hard granite, all I got was wet. It became pretty clear that the storm wasn’t producing any lightning, but as the sky behind me started to brighten while the rain continued falling over Yosemite Valley, I realized that conditions were ripe for a rainbow. Sure enough, long after I would have packed up and headed home had I been focused solely on lightning, this rainbow was my reward.
The moral if my story is that despite all appearances to the contrary, rainbows are not random—when sunlight strikes raindrops, a rainbow occurs, every time. The reason we don’t always see the rainbow not because it isn’t happening, it’s because we’re not in the right place. And that place, geometrically speaking, is always the same. Of course sometimes seeing the rainbow requires superhero ability like levitation or teleportation, but when we’re armed with a little knowledge and anticipation, we can put ourselves in position for moments like this.
I can’t help with the anticipation part, but here’s a little knowledge infusion (excerpted from the Rainbow article in my Photo Tips section).
Energy generated by the sun bathes Earth in continuous electromagnetic radiation, its wavelengths ranging from extremely short to extremely long (and every wavelength in between). Among the broad spectrum of electromagnetic solar energy we receive are ultra-violet rays that burn our skin and longer infrared waves that warm our atmosphere. These wavelengths bookend a very narrow range of wavelengths the human eye sees.
Visible wavelengths are captured by our eyes and interpreted by our brain. When the our eyes take in light consisting of the full range of visible wavelengths, we perceive it as white (colorless) light. We perceive color 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. 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 and scattered by the surface, enabling us to see the water; some light passes through the water’s surface, enabling us to see what’s in the water; and some light is reflected by the surface, enabling us to see reflections.
(From this point on, for simplicity’s sake, it might help to visualize what happens when water strikes a single drop.)
Light traveling from one medium to another (e.g., from air into water) refracts (bends). Different wavelengths refract different amounts, causing the light to split into its component colors. Light that passes through a water refracts (bends). Different wavelengths are refracted different amounts by water; this separates the originally homogeneous white light into the multiple colors of the spectrum.
But simply separating the light into its component colors isn’t enough to create a rainbow–if it were, we’d see a rainbow whenever light strikes water. Seeing the rainbow spectrum caused by refracted light requires that the refracted light be returned to our eyes somehow.
A raindrop isn’t flat like a sheet of paper, it’s spherical, like a ball. Light that was refracted (and separated into multiple colors) as it entered the front of the raindrop, continues through to the back of the raindrop, where some is reflected. Red light reflects back at about 42 degrees, violet light reflects back at about 40 degrees, and the other spectral colors reflect back between 42 and 40 degrees. What we perceive as a rainbow is this reflection of the refracted light–notice how the top color of the primary rainbow is always red, and the bottom color is always violet.
Every raindrop struck by sunlight creates a rainbow. 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 40-42 degree angle at which the raindrop reflects the spectrum of rainbow colors.
Fortunately, viewing a rainbow requires no knowledge of advanced geometry. To locate or anticipate a rainbow, picture an imaginary straight line originating at the sun, entering the back of your head, exiting between your eyes, and continuing down into the landscape in front of you–this line points to the “anti-solar point,” an imaginary point exactly opposite the sun. With no interference, a rainbow would form a complete circle, skewed 42 degrees from the line connecting the sun and the anti-solar point–with you at the center. (We don’t see the entire circle because the horizon gets in the way.)
Because the anti-solar point is always at the center of the rainbow’s arc, a rainbow will always appear exactly opposite the sun (the sun will always be at your back). It’s sometimes helpful to remember that your shadow always points toward the anti-solar point. So when you find yourself in direct 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 circle segment 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 its circle you see and the higher it appears above the horizon; conversely, the higher the sun, the less of its circle is above the horizon and the flatter (and lower) the rainbow will appear.
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 belowthe 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 overhead, 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, the order of the colors is the secondary rainbow is reversed.
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.
Click an image for a closer look and slide show. Refresh the window to reorder the display.