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. How does a lens form an image? See how light rays are refracted by a lens. Watch how the image changes when you adjust the focal length of the lens, move the object, move the lens, or move the screen.
Figure 6. This prism will disperse the white light into a rainbow of colors. The incident angle is Skip to main content. Geometric Optics. Search for:. Dispersion: The Rainbow and Prisms Learning Objective By the end of this section, you will be able to: Explain the phenomenon of dispersion and discuss its advantages and disadvantages.
Dispersion Dispersion is defined to be the spreading of white light into its full spectrum of wavelengths. Making Connections: Dispersion Any type of wave can exhibit dispersion.
Rainbows Rainbows are produced by a combination of refraction and reflection. Click to run the simulation. A beam of white light goes from air into water at an incident angle of At what angles are the red nm and violet nm parts of the light refracted? By how much do the critical angles for red nm and violet nm light differ in a diamond surrounded by air?
These colors are often observed as light passes through a triangular prism. Upon passage through the prism, the white light is separated into its component colors - red, orange, yellow, green, blue and violet.
The separation of visible light into its different colors is known as dispersion. It was mentioned in the Light and Color unit that each color is characteristic of a distinct wave frequency; and different frequencies of light waves will bend varying amounts upon passage through a prism.
In this unit, we will investigate the dispersion of light in more detail, pondering the reasons why different frequencies of light bend or refract different amounts when passing through the prism.
Earlier in this unit, the concept of optical density was introduced. Different materials are distinguished from each other by their different optical densities. The optical density is simply a measure of the tendency of a material to slow down light as it travels through it. As mentioned earlier, a light wave traveling through a transparent material interacts with the atoms of that material.
When a light wave impinges upon an atom of the material, it is absorbed by that atom. The absorbed energy causes the electrons in the atom to vibrate.
If the frequency of the light wave does not match the resonance frequency of the vibrating electrons, then the light will be reemitted by the atom at the same frequency at which it impinged upon it.
The light wave then travels through the interatomic vacuum towards the next atom of the material. Once it impinges upon the next atom, the process of absorption and re-emission is repeated.
The optical density of a material is the result of the tendency of the atoms of a material to maintain the absorbed energy of the light wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance. Thus, while a light wave travels through a vacuum at a speed of c 3. The index of refraction value n provides a quantitative expression of the optical density of a given medium. Materials with higher index of refraction values have a tendency to hold onto the absorbed light energy for greater lengths of time before reemitting it to the interatomic void.
The more closely that the frequency of the light wave matches the resonant frequency of the electrons of the atoms of a material, the greater the optical density and the greater the index of refraction. A light wave would be slowed down to a greater extent when passing through such a material.
What was not mentioned earlier in this unit is that the index of refraction values are dependent upon the frequency of light. For visible light, the n value does not show a large variation with frequency, but nonetheless it shows a variation.
For instance for some types of glass, the n value for frequencies of violet light is 1. The absorption and re-emission process causes the higher frequency lower wavelength violet light to travel slower through crown glass than the lower frequency higher wavelength red light. It is this difference in n value for the varying frequencies and wavelengths that causes the dispersion of light by a triangular prism.
Violet light, being slowed down to a greater extent by the absorption and re-emission process, refracts more than red light. Upon entry of white light at the first boundary of a triangular prism, there will be a slight separation of the white light into the component colors of the spectrum. The amount of overall refraction caused by the passage of a light ray through a prism is often expressed in terms of the angle of deviation.
The angle of deviation is the angle made between the incident ray of light entering the first face of the prism and the refracted ray that emerges from the second face of the prism.
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