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9.12 Dispersion: the rainbow and prisms  (Page 2/4)

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Index of refraction n In selected media at various wavelengths
Medium Red (660 nm) Orange (610 nm) Yellow (580 nm) Green (550 nm) Blue (470 nm) Violet (410 nm)
Water 1.331 1.332 1.333 1.335 1.338 1.342
Diamond 2.410 2.415 2.417 2.426 2.444 2.458
Glass, crown 1.512 1.514 1.518 1.519 1.524 1.530
Glass, flint 1.662 1.665 1.667 1.674 1.684 1.698
Polystyrene 1.488 1.490 1.492 1.493 1.499 1.506
Quartz, fused 1.455 1.456 1.458 1.459 1.462 1.468
Figure (a) shows a triangle representing a prism and a pure wavelength of incident light falling onto it and getting refracted at both sides of the prism. The refracted ray runs parallel to the base of the prism and then emerges after getting refracted from the other surface. Figure (b) shows a triangle representing a prism and an incident white light falling onto it and getting refracted at the first surface with two refracted rays with slightly different angles of separation. The refracted rays, on falling on the second surface, refract with various angles of refraction. A sequence of red to violet is produced when light emerges out of the prism. Red at 760 nanometers and violet at 380 nanometers.
(a) A pure wavelength of light falls onto a prism and is refracted at both surfaces. (b) White light is dispersed by the prism (shown exaggerated). 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.

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 [link] . 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 [link] (a). (There is no dispersion caused by reflection at the back surface, since the law of reflection does not depend on wavelength.) The actual rainbow of colors seen by an observer depends on the myriad of rays being refracted and reflected toward the observer’s eyes from numerous drops of water. 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 [link] (b). (If there are two reflections of light within the water drop, another “secondary” rainbow is produced. This rare event produces an arc that lies above the primary rainbow arc—see [link] (c).)

Rainbows

Rainbows are produced by a combination of refraction and reflection.

Sun light incident on a spherical water droplet gets refracted at various angles. The refracted rays further undergo total internal reflection and when they leave the water droplet, a sequence of colors ranging from violet to red is formed.
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.
In figure (a) sunlight is incident on two water droplets close to one another. The incident rays undergo refraction and total internal reflection. From the first droplet, violet color emerges and from the second, red emerges. A woman observes from a distance, the band of seven colors with red on top and violet at the bottom. Two rays each from red and violet reach the observer’s eyes. The angle of separation between the incident light and the emerging red light is theta. In figure (b), a man looks at the rainbow, which is in the shape of an arc. A parallel beam of blue colored rays fall on the rainbow at different positions and then reaches the observer, each ray making the same angle theta with the incident ray. The rays reaching the observer are red in color. Figure (c) shows a spectacular double rainbow in the sky with white clouds as a backdrop.
(a) Different colors emerge in different directions, and so you must look at different locations to see the various colors of a rainbow. (b) The arc of a rainbow results from the fact that a line between the observer and any point on the arc must make the correct angle with the parallel rays of sunlight to receive the refracted rays. (c) Double rainbow. (credit: Nicholas, Wikimedia Commons)

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.

Section summary

  • The spreading of white light into its full spectrum of wavelengths is called dispersion.
  • Rainbows are produced by a combination of refraction and reflection and involve the dispersion of sunlight into a continuous distribution of colors.
  • Dispersion produces beautiful rainbows but also causes problems in certain optical systems.

Problems&Exercises

(a) What is the ratio of the speed of red light to violet light in diamond, based on [link] ? (b) What is this ratio in polystyrene? (c) Which is more dispersive?
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Source:  OpenStax, Concepts of physics. OpenStax CNX. Aug 25, 2015 Download for free at https://legacy.cnx.org/content/col11738/1.5
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