You’re standing in your backyard after a summer thunderstorm. The air smells like wet pavement and grass. You turn your back to the sun, and there it is—a massive, shimmering arc of color. We call them rainbows in the sun, though technically, you can’t have one without the other. It feels like magic. Honestly, it’s just physics, but the kind of physics that makes you feel small in a good way. Most people think they know how rainbows work because they learned about prisms in third grade. Red, orange, yellow, green, blue, indigo, violet. ROYGBIV. But there is a lot more going on in that light than a simple color chart.
Did you know that no two people ever see the exact same rainbow? It's true. Because a rainbow isn't a physical object located in a specific spot in the sky—like a cloud or a bird—it’s an optical phenomenon that exists only relative to your eyes. If you move an inch to the left, the light hitting your retina comes from different water droplets. Your friend standing next to you is seeing a completely different set of colors. It’s a private show. Just for you.
The Brutal Physics of Rainbows in the Sun
To get a rainbow, you need a very specific "recipe." First, the sun has to be behind you. Always. If you're looking at the sun, you aren't seeing a rainbow; you're just getting blinded. Second, there has to be water in the air in front of you. This is usually rain, but mist from a garden hose or a waterfall works just as well.
The light enters a water droplet, slows down, and bends. This is called refraction. Then, it hits the back of the droplet and bounces off—reflection. Finally, it speeds back up as it exits the droplet, bending again. Because different colors of light travel at different speeds through water, they spread out.
Think of it like a crowded hallway. Blue light is high energy and gets bumped around more, so it bends at a sharper angle. Red light is the "heavy hitter" and stays a bit straighter. This creates that 42-degree angle that is the "sweet spot" for human eyes. If the sun is higher than 42 degrees in the sky, you won't see a rainbow from the ground because the arc is actually below the horizon. That’s why you mostly see rainbows in the sun during the late afternoon or early morning. They are creatures of the golden hour.
It's Not Just a Bow; It's a Circle
Here is the thing that messes with people’s heads: rainbows aren't arcs. They are circles.
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The only reason we see them as "bows" is because the ground gets in the way. If you were in an airplane or standing on the edge of a very tall cliff, you might see a full, 360-degree circle of color. Pilots see them all the time. It’s called a "glory" sometimes, though that’s technically a slightly different atmospheric effect involving diffraction. But a true circular rainbow is a real, documented thing. It's basically the universe showing off.
Why Some Rainbows Look "Off"
Have you ever seen a rainbow that looked washed out? Or one that was incredibly dark?
The size of the water droplets changes everything. Big, fat raindrops from a heavy thunderstorm create vivid, distinct colors. Tiny droplets of mist or fog create what we call "fogbows." These look like ghostly, white arches because the droplets are so small that the light waves interfere with each other and smear the colors together. It’s almost eerie.
Then there’s the "double rainbow." You’ve seen the viral videos. It happens when light reflects twice inside the water droplet. Because of that second bounce, the colors are reversed. In a primary rainbow, red is on the outside. In the secondary one, red is on the inside. Also, pay attention to the space between the two bows. It’s usually darker than the rest of the sky. This is called Alexander’s Dark Band, named after Alexander of Aphrodisias, who first described it in 200 AD. The light that would have been reflected there is being redirected into the bows themselves, leaving a "gap" of light in the middle.
The Myth of Indigo
Let’s talk about Isaac Newton for a second. He’s the guy who gave us the seven-color spectrum. But many modern color scientists argue that indigo shouldn’t even be there. Newton was a bit of a mystic. He believed the number seven was mathematically sacred—seven notes in a musical scale, seven days of the week, seven known planets at the time. So, he squeezed "indigo" in between blue and violet to make the numbers fit his worldview. Most people can’t actually distinguish indigo in a natural rainbow. To us, it just looks like blue fading into purple.
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Spotting Rainbows Like a Pro
If you want to find rainbows in the sun more often, you have to become a bit of a weather nerd. You need "broken" weather. Solid gray overcast skies won't do it because the sun can't get through. You want those days where it's raining but you can see patches of blue sky.
- Check the Sun's Height: If your shadow is longer than you are, you’re in the prime window.
- Look Opposite the Sun: If the sun is in the west (setting), look east.
- Find the "Rain Curtain": Look for areas where the clouds look blurry and grey—that’s where the rain is falling.
If you're really desperate to see one, just grab a garden hose. Set it to a fine mist and stand with your back to the sun. Move the nozzle around until you hit that 42-degree angle. Boom. Instant physics lesson.
Common Misconceptions About Sunlit Rainbows
A lot of people think rainbows only happen in the summer. Nope. You can have "snowbows," though they are incredibly rare because ice crystals usually refract light differently, creating "halos" or "sun dogs" instead of rainbows.
Another big one: "The rainbow is touching that hill over there!"
It isn't.
As we established, a rainbow is a directional light effect. As you move toward the hill, the rainbow moves with you. You can’t get to the end of a rainbow any more than you can catch your own shadow. It’s a mathematical horizon, not a physical destination. This is why the whole "pot of gold" thing is such a cruel joke—the closer you get, the further it stays.
The Role of Pollution
Oddly enough, the quality of your rainbow can tell you something about the air you're breathing. Very bright, red-heavy rainbows often happen when there’s a lot of dust or smoke in the air. This is because smaller wavelengths (blues and greens) get scattered away by the pollutants, leaving only the long-wavelength reds to make it through. If you see a sunset rainbow that looks like it's bleeding deep crimson, there’s a good chance there’s a fire or heavy smog nearby.
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Practical Steps for Photographers and Observers
If you’re trying to capture rainbows in the sun on your phone or a DSLR, there are a few things that actually work.
- Use a Polarizing Filter: This is the single best trick. A polarizer can make the colors of a rainbow pop with almost unbelievable intensity. If you rotate the filter, you can actually make the rainbow disappear entirely and then reappear. This is because rainbow light is highly polarized.
- Underexpose Your Shot: Cameras often try to brighten the sky, which washes out the delicate colors of the bow. Drop your exposure by a stop or two. The colors will look much richer.
- Find a Dark Background: A rainbow against a dark, stormy cloud looks way more dramatic than one against a pale blue sky. Contrast is your friend.
- Wide Angle is Key: A standard lens often can't fit the whole arc. If you're on an iPhone, use the 0.5x lens.
Watching for these optical wonders is a great way to practice "situational awareness." It forces you to look at the world as a 3D space where light and water are constantly interacting. It’s a reminder that even on a rainy day, the sun is still doing its thing in the background.
To get the most out of your next sighting, stop trying to take a selfie with it for a second. Just look. Notice the "supernumerary fringes"—those tiny, faint green and pink bands sometimes visible just inside the primary violet. They are caused by the wave-like interference of light, a phenomenon that helped physicists prove that light isn't just particles, but waves too. All that, just hanging out in the sky after a shower.
Check your local weather app for "scattered showers" combined with "partly cloudy" forecasts. These are the peak conditions for atmospheric optics. Position yourself between the rain and the sun, and keep your eyes on the horizon opposite the light source. With a bit of patience, you'll catch the light exactly when the physics aligns.