Basic concepts
Wave
Generally speaking, a wave is a periodic disturbance that propagates (travels) in space. Waves are tied up with oscillations: many times, if an oscillation is started somewhere in a medium, this oscillation may affect neighbouring regions and become a source of waves. One can think of waves as travelling oscillations.
It is important to note that it is not the particles of the medium that travel during a wave, only the oscillatory state and the energy associated with it. If you throw a pebble into a pond, ripples will form on the surface, but it does not mean that water molecules would move out of the centre with the ripples. Water molecules will pretty much oscillate in place, but the attractive forces between them will tug at neighbouring molecules, causing the oscillatory state to spread.
Example: waves on a rope
| If one pulls taut a rope with one end fixed and starts to move the free end in simple harmonic motion, the oscillations initiated at the free end will travel on towards the fixed end. What makes it possible is the elastic force between neighbouring regions of the rope, which carries the oscillation on to adjacent areas with some delay. |
Types of waves: mechanical v electromagnetic
Mechanical waves
Mechanical waves involve the oscillation of the particles of a medium. Due to the interactions between particles, these oscillations will travel on in the medium. For this very reason, mechanical waves require a medium to propagate: eg, sounds cannot be heard in vacuum since sound is a mechanical wave and as such, it will not travel on in the absence of a medium.
Examples: sound, waves on a rope, earthquakes.
Electromagnetic waves
Electromagnetic waves involve synchronised oscillations of the electric field and the magnetic field in perpendicular planes. Electromagnetic waves originate in the phenomenon of electromagnetic induction: changing magnetic fields generate changing electric fields, which in turn generate changing magnetic fields, &c, giving rise to a self-sustaining process travelling in space. Electromagnetic waves do not require a medium to propagate: eg, sunlight does reach the surface of the Earth, even though there is vacuum in between.
Examples: light, X-rays, gamma rays, radio waves, microwaves.
Types of waves: transverse v longitudinal
Transverse waves
In transverse waves, oscillations occur in a direction perpendicular to that of propagation. For example, the oscillation in a rope wave may be perpendicular whilst the wave travels in the horizontal direction.
Examples: rope wave, electromagnetic waves.
Longitudinal waves
In longitudinal waves, oscillations take place in the same direction as wave propagation. In sound waves, for instance, pressure oscillates in the same direction in which the wave travels.
Examples: sound, lengthwise waves in a slinky.
Crests and troughs. Amplitude.
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CrestA crest is a point in the wave where the displacement from the equilibrium — or the electric or magnetic field for electromagnetic waves — is a maximum. The propagation of the wave is reflected in how the crests change their locations. TroughA trough is a point in the wave where the displacement from the equilibrium — or the electric or magnetic field for electromagnetic waves — is a minimum. The propagation of the wave is reflected in how the troughs change their locations. |
The amplitude of the wave is the maximum displacement from the equilibrium, or the maximum value of the electric or magnetic field in an electromagnetic wave.
Compressions and rarefactions
Compressions
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In longitudinal waves, there are areas where the oscillating quantity — such as the pressure or density for sounds or the density of coils in a slinky — reaches its maximum. We call these locations compressions. The propagation of the wave means that these locations travel in the direction of propagation. |
Rarefactions
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In longitudinal waves, there are areas where the oscillating quantity — such as the pressure or density for sounds or the density of coils in a slinky — has a minimum. We call these locations rarefactions. The propagation of the wave means that these locations travel in the direction of propagation. |