A secondary rainbow appears inverted compared to a primary rainbow because of the way light reflects and refracts within the water droplets.
Here's a breakdown of the differences leading to the inversion:
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Primary Rainbow: Light enters a raindrop, refracts (bends), reflects once off the back of the raindrop, and then refracts again as it exits. This results in the familiar, brighter rainbow with red on the outside and violet on the inside.
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Secondary Rainbow: Light enters a raindrop from the bottom, refracts, reflects twice off the back of the raindrop, and then refracts again as it exits. This double reflection flips the order of the colors compared to the primary rainbow. The colors in a secondary rainbow are reversed, with red on the inside and violet on the outside. Because more light is lost during the two reflections, secondary rainbows are fainter than primary rainbows.
In summary, the inversion results from the different entry point of the light on the raindrop and the additional internal reflection within the water droplet for a secondary rainbow.
