A typical raindrop is also spherical and therefore its effect on sunlight is symmetrical about an imagined axis connecting the center of the drop and the sun. Because of this symmetry, so long as the raindrop is viewed along a line of sight that makes a to degree angle with the direction of the sun, the various colors of light will be visible. Thus, a rainbow is actually a circle centered on the point directly opposite the sun from the observer--the so-called antisolar point--with an angular radius of 42 degrees.
We don't actually see the full circle because the earth gets in the way. The closer the sun is to the horizon, the more of the circle we see. Right at sunset, we would see a full semicircle of a rainbow with the top of the arch 42 degrees above the horizon. The higher the sun is in the sky, the less of a rainbow is visible above the horizon. Sign up for our email newsletter. Already a subscriber?
Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. When it exits the droplet, it makes a rainbow. Sunlight is made up of many wavelengths—or colors—of light. Some of those wavelengths get bent more than others when the light enters the water droplet.
Violet the shortest wavelength of visible light bends the most, red the longest wavelength of visible light bends the least. So when the light exits the water droplet, it is separated into all its wavelengths. The light reflecting back to you, the observer with the Sunlight coming from behind you, from the water droplets will appear separated into all the colors of the rainbow! Violet will be on the bottom and red on the top.
A secondary rainbow appears if the sunlight is reflected twice inside the water droplets. Secondary rainbows are fainter, and the order of the color is reversed, with red on the bottom. Credit: Leonardo Weiss via Wikimedia Commons. Sometimes you can see another, fainter secondary rainbow above the primary rainbow. To red light, which has a long wavelength, the refractive index of water is almost 1.
Sunlight hitting a water droplet sphere in the atmosphere will be refracted on the surface of the droplet, and enters the droplet. When the refraction process occurs, the light breaks up into seven colors inside the water droplet, and is next reflected at the other surface of the droplet after traveling inside it. Note that in reflection the angle of reflection is the same as the angle of incidence, which means that reflected light travels in a predetermined path while maintaining the difference of angle of refraction.
The light is refracted again when it exits the droplet, further emphasizing the dispersion. The primary reflection of a main rainbow and the secondary reflection from a slightly darker auxiliary rainbow disperse the light into the seven colors our eyes see. You can see rainbows when the sun is located right behind you. The main rainbow becomes visible at an angle of around 40" from the horizon.
You can see the auxiliary rainbow at about 53". The orders of the colors reflected from the water droplets in the main rainbow and in the auxiliary rainbow are reversed, as shown in the illustration.
Chapter 1: The Mysteries of Light. Why Is the Sky Blue? How Do Rainbows Form? Why Light Fades in the Bathroom?
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