Principle of Superposition and Stationary Waves
Principle of Superposition
If two or more waves are travelling in a medium at the same time, then the resultant displacement of waves is equal to the vector sum of individual displacement of the wave.
Let \(\vec y_1, \vec y_2, \vec y_3 \dots \dots \vec y_n \) be the displacements of different waves travelling in medium at same time. Then, according to the principle of superposition
Resultant displacement,
$$ \vec y = \vec y_1+\vec y_2+ \vec y_3+\dots \dots +\vec y_n $$
If two waves each of same amplitude ‘a’ superposition the resultant displacement is
\begin{align*} y &= a + a = 2a \: (\text {constructive interference}) \\ \text {and} \\ y &= a  a = 0 \: (\text {destructive interference}) \\ \end{align*}
The principle of superposition can be used to explain many wave phenomena. Some of them are as follows:
 When two waves of same frequency moving in the same direction superpose, constructive interference of waves is produced.
 When two waves of same frequency moving in the opposite direction superpose, stationary waves are produced.
 When two waves of slightly different frequency moving in the same direction superpose, beats are produced.
Stationary Wave
If two waves having same amplitude and frequency are travelling in opposite direction on the medium at the same time then the resultant displacement of the wave becomes zero and the waves come in rest or stationary state. This type of wave is called stationary wave or standing wave. There are different point appear in a stationary wave.
Nodes
Two different waves meet at a point are called nodes. In node particles of the medium are in permanent rest.
Antinodes
The maximum displacement of the wave in either side from equilibrium position is called antinodes. The antinode particles are not in permanent rest.
Expression of Stationary Wave:
Consider two waves having displacement y_{1} and y_{2} travelling in opposite direction with same amplitude and frequency on the medium at same time. Then, the general progressive wave equation for first wave is given by
\begin{align*} \vec y_1 &= a\sin (\omega t – kx) \dots (i) \end{align*} and general progressive wave equation for second wave is given by,\begin{align*}\vec y_2 &= a\sin (\omega t + kx) \dots (ii) \\ \text {where,} \: K &= \frac {2\pi }{\lambda }\\ &= \text {wave number or propagation constant,} \\ \text {According to principle of superposition. } \\ \text {The resultant displacement of two wave} \\ \end{align*}
\begin{align*}\vec y &= \vec y_1 + \vec y_2 \\ \vec y &= a\sin (\omega t – kx) + a\sin (\omega t + kx) \\ &= a[\sin (\omega t – kx) + \sin (\omega t + kx)] \\ &= a[2\sin \frac {(\omega t – kx + \omega t + kx)}{2}] \cos (\frac {\omega t – kx  \omega t – kx}{2}) \\ &= a[2\sin \omega t . \cos (kx)] \\ &= 2a\sin \omega t. \cos kx \\ \vec y &= 2a\cos kx . \sin \omega t \dots (iii) \\ \end{align*}
Equation (iii) gives the expression for stationary wave where, \( 2a\cos kx \)gives the amplitude of the stationary wave and \( \sin \omega t \)gives the nature of oscillation\begin{align*} \text {If} \: 2a\cos kx = 0 \\ kx &= \left (x + \frac 12 \right ) \pi \\ \frac {2\pi x}{\lambda } &= \left (x + \frac 12 \right )\pi \\ \text {or,} \: x &= \left (x + \frac 12 \right ) \frac {\pi }{2} \\ \end{align*}
Equation (iv) represents the position of node where displacement of wave is 0 and n is interger.\begin{align*} \text {If} \: \cos kx &= \pm 1 \\ kx &= n\pi \\ \frac {2\pi }{\lambda } . x &= x\pi \\ \text {or,} \: x &= \frac {n .\lambda }{2} \dots (v) \\ \end{align*}
Equation} (v) represents the position of antinodes of stationary wave where displacement is maximum. If \(\sin \omega t = \pm 1\) Then the wave oscillates in either side of mean position \begin{align*} \text {If} \: \sin \omega t = 0 \\ \text {The wave just crosses the nodes} \\ \end{align*}
Properties of Stationary Wave
 When two progressive waves of same amplitude and frequency travel in a medium in opposite direction to each other, a stationary wave is produced.
 Nodes and antinodes are formed alternatively in the wave. A node is a position of zero displacement and maximum strain whereas an antinode is the position of maximum displacement and zero strain. However, if these waves travel in the same direction, they produce an interference pattern.
 All the particles except at the nodes vibrate simple harmonically with the period equal to that of the wave.
 The amplitude of vibration gradually increases from zero at node to maximum at antinode.
 The medium splits into segments and all the particles of segment vibrate in phase. The particles in one segment have a phase difference of p with the particles in the neighboring segment.
 The disturbance does not travel forward; there is no transfer of energy.
 The disturbance between two adjacent nodes is λ/2 and that between two adjacent antinodes is also λ/2. The distance between a node and adjacent node is λ/4.
 The velocity and acceleration of all the particles separated by a distance λ are the same at the given instant.
Difference between a Progressive and a Stationary Wave
S.N.  Progressive Wave  Stationary Wave 
1.  The disturbance travels forward in a medium and is handed over from one particle to the next after some time.  The disturbance is at rest and does not move at all. So there is no transfer of disturbance to the neighboring particles. 
2.  The amplitude of oscillation is same at all positions in the medium.  The amplitude of oscillation varies from zero at the node to a maximum at the antinode. 
3.  No particle is permanently at rest.  The particles at nodes are permanently at rest. 
4.  Energy is transmitted from particle to particle across every section of the medium.  Energy is not transmitted from particle to particle, i.e. no transfer of energy across every section of the medium. 
5.  As the disturbance moves, every part of the medium suffers a change in density.  At antinodes, there is no change in density but at node there is maximum. 
6.  At every point, there is variation in pressure.  Pressure variation is maximum at nodes and zero at antinodes. 
7.  Regular phase difference exists between successive particles.  All the particles in between two successive nodes are in phase. 
8.  The value of maximum velocity for all particles of the medium is same.  The value of maximum velocity for different particles is different and velocity of the particles at the node is always zero. 
Reference
Manu Kumar Khatry, Manoj Kumar Thapaet al. Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.
S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.
According to superposition principle, if two or more waves are travelling in a medium at the same time, then the resultant displacement of waves is equal to the vector sum of individual displacement of the wave.
If two waves having same amplitude and frequency are travelling in opposite direction on the medium at the same time then the resultant displacement of the wave becomes zero and the waves come in rest or stationary state. This type of wave is called stationary wave or standing wave.
Two different waves meet at a point are called nodes.
The maximum displacement of the wave in either side from equilibrium position is called antinodes.

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