In this way, the wall stops much of the noise, while the shrub diffuses the other part of the noise. One example is placing shrubs that go above the top of the wall. Since the diffraction is caused at the edge of the wall, placing objects or material above that edge can help to diffuse the diffraction. This is a form of diffraction of the sound waves.Įdge diffracts the noise Possible solutions When sound waves reach the edge of an obstacle, that edge causes waves to curve as if the edge was a source of the sound. The problem is that some noise still seems to reach your back yard, due to the diffraction property of sound. A common way to do that is to build a tall fence or wall in an attempt to block out the sound and noise.Ī solid wall or fence can reflect some sound waves and absorb other waves, such that very little noise gets through the wall. Sound has wavelengths on the order of the size of the door and bends around corners (for frequency of 1000 Hz, \lambda=\frac\\, about three times smaller than the width of the doorway).If your house is near a busy freeway, or there is some other source of noise reaching your home, you might want to find a way to block out that noise. What is the difference between the behavior of sound waves and light waves in this case? The answer is that light has very short wavelengths and acts like a ray. When sound passes through a door, we expect to hear it everywhere in the room and, thus, expect that sound spreads out when passing through such an opening (see Figure 5). What happens when a wave passes through an opening, such as light shining through an open door into a dark room? For light, we expect to see a sharp shadow of the doorway on the floor of the room, and we expect no light to bend around corners into other parts of the room. The ray bends toward the perpendicular, since the wavelets have a lower speed in the second medium. Huygens’s principle applied to a straight wavefront traveling from one medium to another where its speed is less. The wavelets closer to the left have had time to travel farther, producing a wavefront traveling in the direction shown.įigure 4. As the wavefront strikes the mirror, wavelets are first emitted from the left part of the mirror and then the right. In addition, we will see that Huygens’s principle tells us how and where light rays interfere.įigure 3 shows how a mirror reflects an incoming wave at an angle equal to the incident angle, verifying the law of reflection. 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 later. These are drawn at a time t later, so that they have moved a distance s = vt. Each point on the wavefront emits a semicircular wave that moves at the propagation speed v. A wavefront is the long edge that moves, for example, the crest or the trough. The new wavefront is a line tangent to the wavelets.įigure 2 shows how Huygens’s principle is applied. ![]() ![]() Each point on the wavefront emits a semicircular wavelet that moves a distance. ![]() ![]() Huygens’s principle applied to a straight wavefront.
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