The effect is most spectacular when the background is dark, as in stormy weather, but can also be observed in waterfalls and lawn sprinklers. (No dispersion occurs at the back surface, because the law of reflection does not depend on wavelength.) The actual rainbow of colors seen by an observer depends on the myriad rays being refracted and reflected toward the observer’s eyes from numerous drops of water. Since the index of refraction of water varies with wavelength, the light is dispersed, and a rainbow is observed ( Figure 1.24(a)). The light is refracted both as it enters and as it leaves the drop. Light enters a drop of water and is reflected from the back of the drop ( Figure 1.23). You may have noticed that you see a rainbow only when you look away from the Sun. Rainbows are produced by a combination of refraction and reflection. In the preceding example, how much distance inside the block of crown glass would the red and the violet rays have to progress before they are separated by 1.0 mm? More technically, dispersion occurs whenever the propagation of light depends on wavelength. Dispersion is defined as the spreading of white light into its full spectrum of wavelengths. This implies that white light is spread out in a rainbow according to wavelength. The sequence of colors in rainbows is the same sequence as the colors shown in the figure. Sunlight, considered to be white, actually appears to be a bit yellow, because of its mixture of wavelengths, but it does contain all visible wavelengths. White light, in particular, is a fairly uniform mixture of all visible wavelengths. The thousands of other hues we can sense in other situations are our eye’s response to various mixtures of wavelengths. When our eye receives pure-wavelength light, we tend to see only one of the six colors, depending on wavelength. These colors are associated with different wavelengths of light, as shown in Figure 1.21. We see about six colors in a rainbow-red, orange, yellow, green, blue, and violet sometimes indigo is listed, too. (credit a: modification of work by “Alfredo55”/Wikimedia Commons credit b: modification of work by NASA) Farisi's contemporary, Theodoric of Freiberg (in Germany), performed similar experiments using other equipment.įigure 1.20 The colors of the rainbow (a) and those produced by a prism (b) are identical. Based on the resulting multicolored spectra of light, he deduced that water droplets split - or decompose - white light into the colors of the rainbow. He filled a large glass vessel with water and placed it inside a camera obscura, in which he could carefully control the entry of light. At that time, there were no microscopes to examine tiny drops of water similar to those in the atmosphere, so Farisi created an enormous drop of water. Working in his native Persia (now Iran), Farisi designed a series of innovative experiments to understand light and clarify the work of earlier scientists. How does sunlight falling on clear drops of rain get broken into the rainbow of colors we see? It is the same process that causes white light to be broken into colors by a clear glass prism or a diamond ( Figure 1.20), and it was examined in detail by Kamal al-Din Hasan ibn Ali ibn Hasan al-Farisi. Summarize the advantages and disadvantages of dispersionĮveryone enjoys the spectacle of a rainbow glimmering against a dark stormy sky.Describe the effects of dispersion in producing rainbows.Explain the cause of dispersion in a prism.By the end of this section, you will be able to:
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