Interference of waves
| The Principle of Superposition states that at any instant,
the wave displacement of the combined motion of any number of
interacting waves at a point is the sum of the displacements of the
component waves at that point. Interference of waves is the superposition of 2 waves
originating from 2 coherent sources. Sources which are coherent produce waves of
the same frequency, amplitude and in phase. Constructive interference occurs
when the crests or troughs of both waves coincide to produce a wave with crests
and troughs of maximum amplitude. Destructive interference occurs when the
crests of one wave coincides with the trough of the other wave, thus canceling
each with the result that the resultant amplitude is zero.
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Interference pattern. An Antinode is a point where constructive interference occurs, whereas a Node is a point where destructive interference occurs. From the picture above, it can be seen that the antinode line (Y) joins all antinodes while the node line joins all nodes (X & Z).
Constructive Interference
Destructive Interference |
Interference of water wave
| Interference of water waves is studied in the ripple
tank. Two spheres vibrating from the same motor will produce two
coherent waves. Interference will occur. If you place a ping pong ball
in the water, and if it is floating on the anti-node, it will bob up
higher. At the node, the ping pong ball will be stationary. |
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Interference of light
  When light beams passes through 2 slits, we would expect to see 2 bright fringes on the screen. But in reality this does not happen. What is seen is like the diagram below (A series of parallel and alternating bright and dark fringes). This is because Diffractions has taken place and the plane waves have become circular waves and interference has occur. Bright fringes are formed when constructive interference (Maxima) takes place while the dark fringes are regions of destructive interference (Minima). Slits S1 & S2 give rise to two coherent light sources. This experiment is obtained from the Young's double-slit experiment.
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 Construction Interference
Destructive Interference Interference of light is more obvious when a monochromatic (1 color) light is used. Because different color lights have their own wavelengths.
Interference Pattern of blue light. Take note that the widths of the bright and dark fringes are the same. This is different from the diffraction pattern where the bright fringes are of different width, broadest in the middle and decreasing in width and intensity as the fringes move away from the middle fringe. (click here to see diffraction pattern) |
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a = distance between two coherent sources/slit
spacing λ = wavelength x = distance between two consecutive antinode lines (or two consecutive node lines). D = distance from the two sources to the point of measurement of x
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Releationship between λ, a, D & x
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Diffraction Grating
Diffraction Grating is a piece of glass with thousands slits. The slits are parallel, very close to one another and are of the same distance away from each other.
Click here to carry out
virtual experiments on Interference.
Interference of Sound waves
| The interference of sound waves occurs when two coherent sound waves interact to produce a pattern of constructive interference (loud sound) and destructive interference (soft sound). | Click here for
simulation
of interference of sound wave. Video clip on sound interference.
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| Application of Interference of sound waves | |
| Destructive Interference in aviation Microphones are used to detect sound in the cabin and the information is fed into a computer. The computer will generate sound that will interfere destructively with the sound in the cabin. The cabin becomes quieter. |
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[Reflection of wave] [Refraction of wave] [Diffraction of wave]
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