5§2-6 standing waves Traveling waves The energy of traveling waves What are standing waves?Standing waves are special cases of wave interference.The superposition of two coherent waves with identical amplitude, frequency and common vibration axis but opposite propagating direction is standing wave.
6y1 and y2 are identicalxyy1The propagating directions are the same.y is Not standing wave!
9Characteristics of standing wave antinodexyEvery element in the mediumvibrates steadily. There is notraveling of waveform.nodeantinode2) The amplitude of standing wave is A(x).3) The positions of maximum amplitude are called antinodes.4) The positions of zero amplitude are called nodes.
10Mathematical expression of standing wave Suppose there are two coherent waves with identical amplitude, frequency and common vibration axis but opposite propagating directions.uxyy1y2uSet the initial phase is 0.
11The standing wave is the superposition of these two coherent waves. using interference of wave, we get:Where:
14The mathematical expression of standing wave is: standing wave equation
15Amplitude of standing wave, A(x) Simple harmonic vibrationStanding wave equation does not satisfy:Therefore, standing wave is different from traveling wave.
16Every element of the medium in standing wave is doing simple harmonic vibration at identical frequency ω.2) But the amplitude of every element might be differentat various position.Since every element of the medium vibrates steadily,the disturbance does not propagate in standing wave.
18xyantinodenodeAdjacent antinodes are separated by a distance of λ/2.Adjacent nodes are also separated by λ/2.The distance between adjacent antinode and node is λ/4.We can measure the distance between two adjacent nodes to determine the wavelength of λ.
19Phase of standing waveIf time is varying, for all the elements in the medium their phases are identical, that is ωt.
20The displacement, velocity and phase of the elements locating in the two sides of one node are opposite.But the displacement, velocity and phase of the elements locating between two nodes are with the same sign.
21§2-7 half-wavelength loss If the incident wave reflects on a certain interface, the phase of the reflected wave is opposite to that of the incident wave.Such phenomena is called half-wavelength loss.Half-wavelength lossAn electromagnetic wave undergoes a phase change of 180o on reflection from a medium of higher reflection index than the one in which it is traveling.
22How can we get the reflected wave with half-wavelength loss? Wave with opposite vibrationBut the reflected wave with half-wavelength loss is not this opposite wave, but this wave with a phase difference of π.
23The conditions which will cause half-wavelength loss: 1) The reflection point is the fixed end of the medium.2) When the wave propagates from a wave thinner medium to a wave denser medium, the reflected wave has half-wavelength loss.Wave denser medium: the medium with a larger reflection index n.Wave thinner medium: the medium with a smaller reflection index n.
24denserthinnerglass, n=1.52Air, n=1Water, n=1.33Water, n=1.33thinnerdenserThe reflected wave hashalf-wavelength lossThe reflected wave hasno half-wavelength lossOnly reflection may have half-wavelength loss possibly. Refraction never has such phenomena.
25Example 2-3-1The wave equation of one wave propagating along x axis can be written as:The reflection occurs at x=0 and the reflection point is one node. Find: 1) the wave equation of the reflected wave. 2) the wave equation of the superposition of these two waves. 3)the position of the nodes and antinodes.
26Solution:1) Because the reflection point is one node, the reflected wave has half-wavelength loss.The wave equation of the reflected wave can be expressed as:The incident wave equation: