This explains why a ray changes direction to become closer to the perpendicular when light slows down. Since the speed of light is smaller in the second medium, the waves do not travel as far in a given time, and the new wavefront changes direction as shown. Each wavelet in the figure was emitted when the wavefront crossed the interface between the media. The law of refraction can be explained by applying Huygens’s principle to a wavefront passing from one medium to another (see Figure 27.7). The direction of propagation is perpendicular to the wavefront, as shown by the downward-pointing arrows. The tangent to these wavelets shows that the new wavefront has been reflected at an angle equal to the incident angle. The wavelets shown were emitted as each point on the wavefront struck the mirror. In addition, we will see that Huygens’s principle tells us how and where light rays interfere.įigure 27.6 Huygens’s principle applied to a straight wavefront striking a mirror. We will find it useful not only in describing how light waves propagate, but also in explaining the laws of reflection and refraction. Huygens’s principle works for all types of waves, including water waves, sound waves, and light waves. The new wavefront is a line tangent to the wavelets and is where we would expect the wave to be a time t t later. These are drawn at a time t t later, so that they have moved a distance s = vt s = vt. Each point on the wavefront emits a semicircular wave that moves at the propagation speed v v. A wavefront is the long edge that moves, for example, the crest or the trough. The new wavefront is a line tangent to all of the wavelets.įigure 27.5 shows how Huygens’s principle is applied. Starting from some known position, Huygens’s principle states that:Įvery point on a wavefront is a source of wavelets that spread out in the forward direction at the same speed as the wave itself. The Dutch scientist Christiaan Huygens (1629–1695) developed a useful technique for determining in detail how and where waves propagate. The direction of propagation is perpendicular to the wavefronts (or wave crests) and is represented by an arrow like a ray. Figure 27.4 A transverse wave, such as an electromagnetic wave like light, as viewed from above and from the side.
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