Question

Why dispersion only occurs by prism??If ans is 60 degree then how it also happens by a water droplet in case of rainbow??

Answer 1

white light strikes a spherical raindrop at all angles of incidence, from 0 degrees at the center to 90 degrees at the edges.
 Just like a glass window, some light reflects, and some enters the drop. The light that enters bends toward the center of drop; the light near the edges bends more, but is always less than the critical angle. The light strikes the back of the drop at the same angle it entered, which is always less than the critical angle. Some reflects, and some exits (bending again). This repeats for the reflected light again, and again, and again.The light that exits after reflecting once forms the primary rainbow. It emerges from the drop at all angles between 0 and about 42 degrees relative to the original sunlight, like the beam of a wide flashlight. But it is much brighter in the outer 0.5 degree or so of this beam. This is what half of causes the rainbow.Because of dispersion, the exact location of this bright band varies by color; the red beam is 42 degrees wide, but the violet one is only 40 degrees wide. This is the other half.When you look at the red band of a rainbow, you are looking at raindrops where you are only in their red beam. When you look at the violet band, you are in the beam of all of the colors, but the violet light is much brighter so that is what you perceive.The sky appears whiter inside the colored bands, than outside, because the dimmer light of all colors combines to appear white.



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