Then the light bounces off the back of the water droplet and goes back the way it came, bending again as it speeds up when it exits the water droplet. Light enters a water droplet, bending as it slows down a bit going from air to denser water. The light reflects off the inside of the droplet, separating into its component wavelengths—or colors. 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. The primary rainbow is caused from one reflection inside the water droplet.
This is why the secondary rainbow appears above the primary rainbow. More technically, dispersion occurs whenever there is a process that changes the direction of light in a manner that depends on wavelength. Dispersion, as a general phenomenon, can occur for any type of wave and always involves wavelength-dependent processes.
Figure 2. Even though rainbows are associated with seven colors, the rainbow is a continuous distribution of colors according to wavelengths. Refraction is responsible for dispersion in rainbows and many other situations.
The angle of refraction depends on the index of refraction, as we saw in The Law of Refraction. We know that the index of refraction n depends on the medium. But for a given medium, n also depends on wavelength.
See Table 1. Note that, for a given medium, n increases as wavelength decreases and is greatest for violet light. Thus violet light is bent more than red light, as shown for a prism in Figure 3b, and the light is dispersed into the same sequence of wavelengths as seen in Figure 1 and Figure 2.
Figure 3. Since the index of refraction varies with wavelength, the angles of refraction vary with wavelength. A sequence of red to violet is produced, because the index of refraction increases steadily with decreasing wavelength. Any type of wave can exhibit dispersion.
Sound waves, all types of electromagnetic waves, and water waves can be dispersed according to wavelength. Dispersion occurs whenever the speed of propagation depends on wavelength, thus separating and spreading out various wavelengths. Dispersion may require special circumstances and can result in spectacular displays such as in the production of a rainbow. This is also true for sound, since all frequencies ordinarily travel at the same speed. If you listen to sound through a long tube, such as a vacuum cleaner hose, you can easily hear it is dispersed by interaction with the tube.
Dispersion, in fact, can reveal a great deal about what the wave has encountered that disperses its wavelengths. The dispersion of electromagnetic radiation from outer space, for example, has revealed much about what exists between the stars—the so-called empty space. Figure 4. Part of the light falling on this water drop enters and is reflected from the back of the drop. This light is refracted and dispersed both as it enters and as it leaves the drop.
Rainbows are produced by a combination of refraction and reflection. You may have noticed that you see a rainbow only when you look away from the sun. Light enters a drop of water and is reflected from the back of the drop, as shown in Figure 4.
The light is refracted both as it enters and as it leaves the drop. Since the index of refraction of water varies with wavelength, the light is dispersed, and a rainbow is observed, as shown in Figure 5a. There is no dispersion caused by reflection at the back surface, since the law of reflection does not depend on wavelength.
The effect is most spectacular when the background is dark, as in stormy weather, but can also be observed in waterfalls and lawn sprinklers. The arc of a rainbow comes from the need to be looking at a specific angle relative to the direction of the sun, as illustrated in Figure 5b.
This rare event produces an arc that lies above the primary rainbow arc—see Figure 5c. Figure 5. Dispersion may produce beautiful rainbows, but it can cause problems in optical systems. White light used to transmit messages in a fiber is dispersed, spreading out in time and eventually overlapping with other messages.
Since a laser produces a nearly pure wavelength, its light experiences little dispersion, an advantage over white light for transmission of information. In contrast, dispersion of electromagnetic waves coming to us from outer space can be used to determine the amount of matter they pass through. As with many phenomena, dispersion can be useful or a nuisance, depending on the situation and our human goals.
0コメント