now let us use huygens's principle and figure 35.19 to derive Snell'a law of refraction. we focus our attention on the instant ray 1 strikes the surface and the subsequent time interval untill ray 2 strikes the surface. During this time interval, the wave at A sends out a huygens waveler (the are centered on A) toward D. In the same time interval. the wave at B sends out a huygens wavelet (the arc centered on B) toward C. Because these two wavelets travel through different media, the radii of wavelets are different. the radius of the wavelet from A is AD=v. where v is the wave speed in the second medium. the radius of the wavelet from B is BC=v. where v is the wave speed in the original medium