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Therefore, . ! .+ 90/ # & 1 ! 90/ # &+ 1 and & 1 ! &+ 1 which is the law of reflection. Now let us use Huygens’s principle and Figure 35.19 to derive Snell’s law of refrac- tion. We focus our attention on the instant ray 1 strikes the surface and the subsequent 2 % t, where v 2 is the wave speed in the second medium. The radius of the wavelet from B is BC ! v 1 % t, where v 1 is the wave speed in the original medium. From triangles ABC and ADC, we find that If we divide the first equation by the second, we obtain But from Equation 35.4 we know that v 1 ! c/n 1 and v 2 ! c/n 2 . Therefore, n 1 sin & 1 ! n 2 sin & 2 which is Snell’s law of refraction. 35.7 Dispersion and Prisms An important property of the index of refraction n is that, for a given material, the dispersion. Because n is a function of wavelength, Snell’s law of refraction indicates that light of different wavelengths is bent at different As we see from Figure 35.20, the index of refraction generally decreases with increasing wavelength. This means that violet light bends more than red light does when angle of deviation. Now suppose that a beam of white light (a combination of all visible wavelengths) is incident on a prism, as illustrated in Figure 35.22. The rays that emerge spread out in a visible spectrum. These colors, in order of decreasing wavelength, are red, orange, yellow, green, blue, and violet. Clearly, the angle of The dispersion of light into a spectrum is demonstrated most vividly in nature by the formation of a rainbow, which is often seen by an observer positioned between the Sun sin & 1 sin & 2 ! c/n 1 c/n 2 ! n 2 n 1 sin &
1 sin &
2 ! v
1 v
2 sin &
1 ! BC ! v
1 %t AC
and
sin &
2 ! AD AC ! v
2 %t AC S E C T I O N 3 5 . 7 • Dispersion and Prisms 1109 1 θ C B 1 θ 1 2 2 θ 2 θ A D 1.54 1.52 1.50 1.48 1.46 400 500 600 700 n Fused quartz Acrylic Crown glass , nm λ Figure 35.19 Huygens’s construc- tion for proving Snell’s law of refraction. At the instant that ray 1 strikes the surface, it sends out a Huygens wavelet from A and ray 2 sends out a Huygens wavelet from B. The two wavelets have different radii because they travel in differ- ent media. Figure 35.20 Variation of index of refraction with vacuum wavelength for three materials. Figure 35.21 A prism refracts a single-wavelength light ray through an angle 3 . δ |