When light passes from a less dense to a more dense medium, as it does when it enters a prism, it splits into its component wavelengths. These recombine when the light exits the prism, and if the two prism faces are parallel, an observer sees white light emerge.
Actually, on closer inspection, a thin red line and a thin violet one are visible. They are evidence of slightly different angles of dispersion caused by the slowing down of the light beam in the prism material. When the prism is triangular, the angles of incidence as the beam enters and leaves the prism are different, so the angles of refraction are also different. When you hold the prism at the proper angle, you can see the spectrum formed by the individual wavelengths. The difference between the angle of the incident beam and that of the emergent beam is called the angle of deviation.
This angle is essentially zero for all wavelengths when the prism is rectangular. When the faces aren't parallel, each wavelength emerges with its own characteristic angle of deviation, and the bands of the observed rainbow increase in width with increasing distance from the prism.
You've no doubt seen a rainbow, and you may be wondering why you can only see them when the sun is behind you and you're at a particular angle to the clouds or to a rain shower. Light does refract inside a water droplet, but if that were the whole story, the water would have be between you and the sun, and that's not what typically happens.
Unlike prisms, water droplets are round. Some of the light reflects inside the water droplet and emerges from the same side of the droplet. That's the light that produces the rainbow.
The light from the sun has a downward trajectory. Light can exit from any part of the raindrop, but the greatest concentration has an angle of deviation of about 40 degrees. The collection of droplets from which light emerges at this particular angle form a circular arc in the sky. If you were able to see the rainbow from an airplane, you would be able to see a complete circle, but from the ground, half the circle is cut off and you only see the typical semicircular arc.
Chris Deziel holds a Bachelor's degree in physics and a Master's degree in Humanities, He has taught science, math and English at the university level, both in his native Canada and in Japan.
He began writing online in , offering information in scientific, cultural and practical topics. His writing covers science, math and home improvement and design, as well as religion and the oriental healing arts. 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? 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. The refraction of light at the entry location into the rectangular glass prism would cause a little separation of the white light. However, upon exiting the glass prism, the refraction takes place in the opposite direction. The light refracts away from the normal, with the violet light bending a bit more than the red light. Unlike the passage through the triangular prism with non-parallel sides, there is no overall angle of deviation for the various colors of white light.
Both the red and the violet components of light are traveling in the same direction as they were traveling before entry into the prism. There is however a thin red fringe present on one end of the beam and thin violet fringe present on the opposite side of the beam.
This fringe is evidence of dispersion. Because there is a different angle of deviation of the various components of white light after transmission across the first boundary, the violet is separated ever so slightly from the red.
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