![]() The diagram below shows the reflected wavefronts and the reflected ray. Upon reaching the barrier placed within the water, these waves bounce off the water and head in a different direction. The blue arrow is called a ray and is drawn perpendicular to the wavefronts. The direction that these wavefronts (straight-line crests) are traveling through the water is represented by the blue arrow. The diagram at the right depicts a series of straight waves approaching a long barrier extending at an angle across the tank of water. These waves will travel through the water until they encounter an obstacle - such as the wall of the tank or an object placed within the water. As viewed on the sheet of paper below the tank, the crests are the dark lines stretching across the paper and the troughs are the bright lines. These straight waves have alternating crests and troughs. If a linear object attached to an oscillator bobs back and forth within the water, it becomes a source of straight waves. Ripple tank demonstrations are commonly done in a Physics class in order to discuss the principles underlying the reflection, refraction, and diffraction of waves. As the waves encounter obstacles in their path, their behavior can be observed by watching the movement of the dark and bright spots on the sheet of paper. As the water waves move through the ripple tank, the dark and bright spots move as well. So the bright spots represent wave troughs and the dark spots represent wave crests. A crest of water will absorb more light than a trough. A portion of light is absorbed by the water as it passes through the tank. A light typically shines upon the water from above and illuminates a white sheet of paper placed directly below the tank. A ripple tank is a large glass-bottomed tank of water that is used to study the behavior of water waves. ![]() The study of waves in two dimensions is often done using a ripple tank. But what if the wave is traveling in a two-dimensional medium such as a water wave traveling through ocean water? Or what if the wave is traveling in a three-dimensional medium such as a sound wave or a light wave traveling through air? What types of behaviors can be expected of such two- and three-dimensional waves? Specifically, there will be some reflection off the boundary and some transmission into the new medium. Rather, a wave will undergo certain behaviors when it encounters the end of the medium. The wave doesn't just stop when it reaches the end of the medium. 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.Previously in Lesson 3, the behavior of waves traveling along a rope from a more dense medium to a less dense medium (and vice versa) was discussed. 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. ![]()
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