You only see one rainbow at a time because the rainbow you see is a unique perspective created by specific raindrops reflecting sunlight directly into your eyes.
Even though countless raindrops are reflecting and refracting sunlight, creating the potential for many rainbows, you can only perceive the light from the set of raindrops that are positioned at the correct angle (approximately 42 degrees relative to the direction of sunlight) to send those specific colored light rays directly to you. This is why each observer sees their own slightly different rainbow.
Here's a breakdown:
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Sunlight and Raindrops: Sunlight enters raindrops, is refracted (bent), then reflects off the back of the raindrop, and is refracted again as it exits. This process separates the white light into its constituent colors, like a prism.
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The 42-Degree Angle: The most intense light is reflected at an angle of about 42 degrees relative to the incoming sunlight. This means that you'll only see the rainbow colors from raindrops located at that specific angle in relation to your position and the sun.
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Your Unique View: Because your position is unique, the raindrops that are positioned at the correct 42-degree angle to send light to your eyes will be different from the raindrops that are positioned to send light to someone standing next to you. That person sees their own rainbow, created by a slightly different set of raindrops.
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Why One Primary Rainbow is Common: While double rainbows can occur (with a fainter, reversed-color secondary rainbow), the intensity of the primary rainbow dominates because the light undergoes one internal reflection within the raindrop.
Therefore, the rainbow you see is a personal phenomenon created by a dedicated set of raindrops just for you, leading to the perception of only one primary rainbow at a time.
