![]() 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. Since different colors diffract by different amounts, white light seen through a diffraction grating will spread out into its component colors as shown in this YouTube of incandescent and florescent diffraction.Figure 27.4 A transverse wave, such as an electromagnetic wave like light, as viewed from above and from the side. A diffraction grating is a piece of glass or plastic with a series of very small grooves, each of which acts like a slit.Why is the light pattern complicated instead of a simple spot? What is the difference in the light pattern between the single slits and the double slits? Finally the laser is shone through a series of double slits. Then the laser is shone through single openings of different sizes. The first is a square opening, the second a hexagonal opening. A red laser beam is shone through several different small openings. Notice that the plane waves on the right spread out into a circle on the left after passing through the small opening. You are looking down onto the surface of a tray of water. Here water waves travel through an opening about the same size as the wavelength and change their direction. Likewise sunsets are orange because when the sun is on the horizon the path the light travels to reach us passes through more atmosphere and even more violet/blue is removed. The sun looks a little more yellow than it really is because the violet/blue part of the spectrum has been removed (scattered out in other directions). Violet and a little blue light is scattered but since our eyes are not as sensitive to violet we see the blue. The sky is blue because clusters of nitrogen and oxygen molecules (which make up most of the atmosphere) have resonances at the same frequency of violet light. The wave is first absorbed and then re-emitted in all directions (or sometimes perpendicular to the incident direction). Scatteringis a similar phenomenon that occurs when a wave interacts with an object that has a resonance frequency the same as the wave frequency. We only notice diffraction when the opening or object is close to the size of the wavelength, so to see diffraction of light it needs to pass through a much smaller opening than a doorway.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |