![]() ![]() Each point on the wave front emits a semicircular wave that moves at the propagation speed v. A wave front is the long edge that moves, for example, with the crest or the trough. The new wave front is tangent to all of the wavelets.įigure 1.26 shows how Huygens’s principle is applied. Starting from some known position, Huygens’s principle states that every point on a wave front 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 wave fronts (or wave crests) and is represented by a ray. The view from above is perhaps more useful in developing concepts about wave optics.įigure 1.25 A transverse wave, such as an electromagnetic light wave, as viewed from above and from the side. The side view would be a graph of the electric or magnetic field. From above, we view the wave fronts (or wave crests) as if we were looking down on ocean waves. A light wave can be imagined to propagate like this, although we do not actually see it wiggling through space. Huygens’s principle is an indispensable tool for this analysis.įigure 1.25 shows how a transverse wave looks as viewed from above and from the side. This is particularly true when the wavelength is not negligible compared to the dimensions of an optical device, such as a slit in the case of diffraction. However, some phenomena require analysis and explanations based on the wave characteristics of light. So far in this chapter, we have been discussing optical phenomena using the ray model of light. Use Huygens’s principle to explain diffraction.Use Huygens’s principle to explain the law of refraction.Use Huygens’s principle to explain the law of reflection.The waves interfere with each other so that there is constructive interference in some areas (left-hand picture) and destructive interference in other areas (right-hand picture).By the end of this section, you will be able to: In the image below, two sources – labelled Sound 1 and 2 – are aligned one above the other. When the same pitch or frequency sound wave is produced from two sources, a pattern of interference is produced. Sound waves and pitchīecause sound travels outwards from a central source, waves interact in interesting patterns. A sound wave with the beat pattern in diagram D will have a volume that varies at a regular rate – you can hear a pulse or flutter in the sound. The resulting wave has points of constructive interference and destructive interference. When we hear the sound of two different musical notes, as shown in diagram C, we hear a complex waveform we think of as harmony.ĭiagram D shows beats – when two sound waves are nearly the same frequency but slightly different. The result of any combination of sound waves is simply the addition of the various waves. They detect the sounds coming into the ear and produce sounds with equal volume but with the peaks and troughs reversed, resulting in near silence. Noise-cancelling headphones work on this principle. The result is a cancellation of the waves. The result is a wave that has twice the amplitude of the original waves so the sound wave will be twice as loud.ĭestructive interference is when similar waves line up peak to trough as in diagram B. With constructive interference, two waves with the same frequency and amplitude line up – the peaks line up with peaks and troughs with troughs as in diagram A above.
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