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. Answered Views. Solutions :. Balendu Anand. Explanation The phenomenon of splitting white light into its components colours violet, indigo, blue, green, yellow, orange and red while passing through a prism is known as the Dispersion of light.
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Company About Us. 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. Because of the different indices of refraction for the different wavelengths of visible light, the angle of deviation varies with wavelength.
Colors of the visible light spectrum that have shorter wavelengths BIV will deviated more from their original path than the colors with longer wavelengths ROY. The emergence of different colors of light from a triangular prism at different angles leads an observer to see the component colors of visible light separated from each other. Of course the discussion of the dispersion of light by triangular prisms begs the following question: Why doesn't a square or rectangular prism cause the dispersion of a narrow beam of white light?
The short answer is that it does. The long answer is provided in the following discussion and illustrated by the diagram below. Suppose that a flashlight could be covered with black paper with a slit across it so as to create a beam of white light.
And suppose that the beam of white light with its component colors unseparated were directed at an angle towards the surface of a rectangular glass prism. As would be expected, the light would refract towards the normal upon entering the glass and away from the normal upon exiting the glass.
But since the violet light has a shorter wavelength, it would refract more than the longer wavelength red light.
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