Double RainbowAlyssa ChuAward: Top 100
School: rye country day school When I visited Niagara Falls, on my walk back along the Niagara River, I spotted a spectacular sight – a double rainbow. Rainbows are created when water droplets in the air disperse and reflect light. Hence, rainbows are commonly seen around waterfalls. Specifically, since water has a different optical density, when sunlight enters a water drop, it slows down and refracts, then is reflected, and finally refracts again upon exiting and speeding up. The colors appear separated – dispersed – since light with shorter wavelengths refract more. A double rainbow occurs when light reflects twice in a water drop. In the second reflection, some of the light exits, making secondary rainbows fainter than primary rainbows. This also inverts the order of the colors in the spectrum. In the sunlight, the impact parameter, referring to where light rays enter a droplet, is uniformly distributed. This means that most light exits where the scattering angle changes the least in regard to the impact parameter – where the magnitude of the derivative is minimized, which occurs at the minima/maxima. The minimum scattering angle after one reflection is around 138˚; the maximum after two is around 130˚. Rainbows are thus seen at these angles. Moreover, in a phenomenon called "Alexander's Band, " the region between the two rainbows (from 130˚ to 138˚) appears darker. This is because no light rays that are reflected once or twice are refracted at such angles and reach the camera directly from droplets in that region. 
|
|