Because each person's horizon is a little different, no one actually sees a full rainbow from the ground. Viewers on the ground can only see the light reflected by raindrops above the horizon. Viewers in aircraft can sometimes see these circular rainbows.
The antisolar point is the center of the circle. Saltwater has a higher refractive index than freshwater, for instance, so rainbows formed by sea spray will be smaller than rainbows formed by rain. A droplet with a high refractive index will help produce a rainbow with a smaller radius. A refractive index is the measure of how much a ray of light refracts (bends) as it passes from one medium to another-from air to water, for example. The radius of a rainbow is determined by the water droplets' refractive index. As this reflected light leaves the droplet, it is refracted again, at multiple angles. It is then reflected by the back of the droplet. Light entering a water droplet is refracted. A refracted wave may appear "bent," while a reflected wave might seem to "bounce back" from a surface or other wavefront. Both refraction and reflection are phenomena that involve a change in a wave's direction. Rainbows are the result of the refraction and reflection of light. In fact, the center of a primary rainbow is the antisolar point, the imaginary point exactly opposite the sun. The sun or other source of light is usually behind the person seeing the rainbow. The appearance of a rainbow depends on where you're standing and where the sun (or other source of light) is shining. Rainbows can also be viewed around fog, sea spray, or waterfalls.Ī rainbow is an optical illusion-it does not actually exist in a specific spot in the sky. The most familiar type rainbow is produced when sunlight strikes raindrops in front of a viewer at a precise angle (42 degrees). A rainbow is a multicolored arc made by light striking water droplets.
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