physics

physics

1. waves in communication

   • Waves generally start with a disturbance – for example, wind blowing across the surface of the sea, or a stone being thrown into a pond. The energy imparted by that disturbance causes a regular repeating motion, backwards and forwards or up and down, which is called an oscillation.
   • Examples of oscillations are the pendulum of a clock, a child on a swing, a weight bouncing on a spring. Often, as in the suspension of a car, our aim is to try to damp down the oscillations as soon as possible. These oscillations are not themselves waves because they do not travel anywhere, but like waves they do have a frequency and periodic time, which describe the rate at which the oscillation repeats itself.
   • In an oscillation, something is displaced from its rest position, but it also has a tendency to bounce back. In a physical oscillation, like the examples above, the displacement is a distance moved by something from rest. But, for example, in an electrical oscillation, the displacement would be a change of voltage or of current going regularly up and down in value. In either case, we measure the size of an oscillation by its amplitude.
   • Waves transfer energy from one place to another, but without causing any net movement of material. The energy transfer depends on the way an initial oscillating system is connected to its surroundings. If that connection can carry some energy from that first oscillation and transfer it to a similar system next to it, then that system will also start oscillating. However, that second oscillation will not be quite in time with the first one.

2. transverse and longitudinal

   • When the displacement occurs in the same direction that the wave travels, for example, in a sound wave, it is a longitudinal wave. By contrast, in a transverse wave the displacement is at right angles to the direction of propagation of the wave, for example, water ripples and electromagnetic waves
   • Transverse waves are easy to picture because they look like the sine wave graphs you normally draw, with displacement on the vertical axis and the distance travelled by the wave plotted horizontally.
   • However, in a longitudinal wave, the different displacements of particles along the direction in which the wave is propagating, lead to a series of compressions (where particles are packed closer together) and rarefactions (where they are further apart). You can create a longitudinal wave in a spring by making it oscillate along its length . A soft ‘slinky’ spring is best for seeing clearly the compressions and rarefactions that travel down its length.
   • Because of this, sound and other longitudinal waves are sometimes described as pressure waves: oscillations in pressure travelling through a solid or fluid medium
   • Earthquakes and other seismic events below the earth’s surface generate two types of shock wave: a longitudinal ‘pressure’ wave and a transverse ‘shaking’ wave. They travel at different speeds and so will each arrive at different times, making earthquakes quite complex events to study.

3. diffraction

   • If the advancing wave-fronts encounter a flat obstacle in front of them, like a wall, most of the wave’s energy is either absorbed or reflected by the wall. However, if the obstacle has edges or gaps, wave energy can travel round the edges or through the gaps. It is then that you may notice diffraction occurring. Although after going through a gap much of the wave energy does keep moving forwards, some of it spreads out in other directions.
   • A good way to see this effect is to use water ripples generated in a specially designed glassbottomed tray (a ‘ripple tank’). By shining a light downwards through the water in the tray onto a horizontal white screen or sheet of paper, the moving ripples can be clearly seen as bright lines. If obstacles are introduced with edges, or with gaps about whose size is a few times the wavelength of the ripples, you can observe diffracted ripples with curved wavefronts, even though the original ripples had straight-line wave-fronts.
   • Every point along a wave-front has oscillations and energy. So each point on the wavefront can act as a secondary source of circular ripples spreading out in all directions. In this way, all the little secondary ripples add together to make a straight (or plane) wave-front keep moving forward as a straight line, until it meets an obstacle.

4. gratings

   • A diffraction grating is a flat plane object. It has a series of regular lines formed on it that block parts of an advancing wave-front. For microwaves, these lines could be a series of regularly spaced metal bars or wires. For light you would usually use a piece of glass with a series of very fine and regularly spaced scratches on its surface. You can get a similar effect for X-rays by using the regularly spaced rows of atoms or molecules in a crystal.
   • When a wave-front meets a diffraction grating, some of the wave’s energy continues propagating forward through the gaps between the grating lines. This is transmission. Some more of the wave’s energy may be absorbed in the grating itself, but the remainder of the energy is scattered backwards as a reflection.
   • The grating creates a set of regularly spaced secondary sources, where each gap in the grating allows energy through. Each gap generates a set of circular wave-fronts spreading out from that location. The spacing, d, between the grating lines is very important. If it is close in size to the wavelength, λ, of the incident radiation (the incoming waves) then the grating will produce an interference pattern of regularly spaced bright and dark lines (fringes). These correspond to strong intensity created at certain angles after diffraction and no wave energy at all at certain other intermediate angles.
   • The interference pattern is due to superposition of the waves from the separate, regularly spaced sources, coming through the gaps in the diffraction grating. Wherever the path difference between waves from adjacent sources works out to be a whole number, n, of wavelengths, then the displacements due to waves from all the separate sources will be in phase. They add together constructively to give a bright spot of high intensity of radiation at that point. The same effect can be clearly demonstrated with water ripples

5. musical instruments

   • The resonator will also have a mechanism for interacting with and absorbing travelling wave energy from outside itself. Small amounts of energy collected over a period of time can be stored up in the stationary wave and build up a much larger amplitude oscillation. This effect is resonance. It happens when the wave energy coming in from outside has a forcing frequency equal or very close to a natural frequency of the resonator.
   • A resonator can be set into stationary wave motion by a sudden impact that transfers a large amount of energy in an instant, which is then stored in the wave. This is the case for a gong or bell. Alternatively, resonators can be excited continuously by an external vibration, which is the case for most musical instruments other than percussion instruments.
   • Radio and TV antennas have a reflector element that bounces the incoming waves back and creates a stationary wave pattern. The detector is placed at an antinode position for the particular wavelength of radiation the aerial has been designed to pick up. In microwave ovens, stationary wave patterns, caused by reflections from the metal sides of the oven, cause hot and cold spots corresponding to antinodes and nodes, hence the need for a turntable. Bound electrons in atoms and molecules behave like stationary waves, bouncing around in the space they are restricted to by the attraction of the nuclear positive charge. The discrete energy levels that electrons can occupy each correspond to a stationary wave pattern. Wave patterns with higher numbers of nodes correspond to higher energy levels.