![]() 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 all of the wavelets.”įigure 17.4 shows how Huygens’s principle is applied. Huygens’s principle states, “Every point on a wavefront is a source of wavelets that spread out in the forward direction at the same speed as the wave itself. He used wavefronts, which are the points on a wave’s surface that share the same, constant phase (such as all the points that make up the crest of a water wave). ![]() The Dutch scientist Christiaan Huygens (1629–1695) developed a useful technique for determining in detail how and where waves propagate. Although wavelengths change while traveling from one medium to another, colors do not, since colors are associated with frequency. It follows that the wavelength of light is smaller in any medium than it is in vacuum. Where λ λ is the wavelength in vacuum and n is the medium’s index of refraction. As it is characteristic of wave behavior, interference is observed for water waves, sound waves, and light waves. Here we see the beam spreading out horizontally into a pattern of bright and dark regions that are caused by systematic constructive and destructive interference. Passing a pure, one-wavelength beam through vertical slits with a width close to the wavelength of the beam reveals the wave character of light. The laser beam emitted by the observatory represents ray behavior, as it travels in a straight line. In Figure 17.2, both the ray and wave characteristics of light can be seen. Interference is the identifying behavior of a wave. ![]() However, when it interacts with smaller objects, it displays its wave characteristics prominently. As is true for all waves, light travels in straight lines and acts like a ray when it interacts with objects several times as large as its wavelength. The range of visible wavelengths is approximately 380 to 750 nm. This is because we are too near the slit for the simplification offered by the parallel-ray approximation to be valid.Where c = 3.00 × 10 8 c = 3.00 × 10 8 m/s is the speed of light in vacuum, f is the frequency of the electromagnetic wave in Hz (or s –1), and λ λ is its wavelength in m. Even though diffraction (through an opening) is fundamentally single-slit interference, the single-slit formula we derived earlier are not accurate for these drawings. Strictly speaking, they are what I call artists’ impressions of the diffraction phenomena because the actual picture is a lot more complicated. The following are typical illustrations of diffraction, found in many textbooks. So the diffraction of sounds waves and water waves as they interact with common objects are much more readily observable. Sound waves and water waves have wavelengths ranging from centimeters to meters. This is why the diffraction of light usually requires a careful and deliberate setup before they are observable. The first minima angle is practically zero, meaning no spreading). So even a 1 mm slit would be too wide to cause any appreciable spreading in the emergent beam. Light waves have very short wavelength (<750 nm for visible light). The diffraction grating is called a diffraction grating because the slits are so narrow that light diffracts extensively after passing through it. We have already discussed the diffraction of light extensively the single slit interference is the diffraction of light (after passing through a slit). The degree of spreading depends on the width of the opening or obstacle relative to the wavelength (of the wave).
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