How Do Rainbows Form?

How do rainbows form?

2-24-2007

One of the most fascinating things occurring in nature, is the formation of a rainbow. Everyone, at one time or another has seen a rainbow, and without a doubt, has wondered why these beautiful things are not seen all the time? This is exactly what will be explained in this paper.

A rainbow, simply put, is the dispersion of white light (from the sun), through a prism (raindrops) into a visible spectrum. To see the rainbow the viewer must be between the raindrops and the sun. The sunlight will bounce off the raindrops and bounce into your eyes.

Although, sunlight is primarily white light, the colors present in a rainbow become visible because the sunlight reflected and also refracted and dispersed all while inside the raindrops. The raindrop acts like a prism, splitting the white light from the sun into visible colors. The raindrops also act like a mirror and reflect some of the refracted light back toward the sun. This is the light we see as a rainbow. The light hits the raindrop and is dispersed or split like going through a prism. Then, the separated colors are reflected off the backside of the drop and then exit where they are refracted again, as the index of refraction index changes between water and air.

Each raindrop contributes only one color to the rainbow that you see. In looking at a rainbow, you are seeing the collective picture made by millions of raindrops. (figure 1)

figure 1.

The raindrops that are lower in the sky contribute the darker colors of the rainbow (blue, indigo, violet and green), while the higher drops add the lighter colors (red, orange, yellow). This is because the colors exit the raindrop at different angles. For instance, red light emerges at 42o, while blue light appears at 40.6o relative to the incoming ray of sunlight. (figure 2.)

Figure 2.

Another mystery regarding rainbows is why do they form in the shape of bows at all? To fully understand this we must define what is known as the anti-solar point. The anti-solar point is the point, relative to the viewer, that is 1800 away from the sun. Basically this is the point where the shadow of your head is cast on the ground. If the sun is up in the sky, your anti-solar point is below the horizon, if the sun has set, your anti-solar point is above the horizon. (figure 3)

figure 3.

The anti-solar point is what is used to determine where a rainbow can be expected to form. Since the colored light that exits from the raindrops does so at specific angles, the anti-solar point is used to measure the angle. This anti-solar point gives a viewer-specific spot in the sky that is most probable