  # Interference   A single point creates waves with concentric circles of light and dark bands. This picture does not show an interference pattern.  It is simply the concentric waves of two points sources drawn in the same plane.  Contrast this with the image to the right This is the interference pattern of two waves.  The two waves add or subtract to form the light and dark regions of the interference pattern Waves are not simple two dimensional objects.  When they interfere with each other, peaks and valleys are formed.  Many interference patterns look like two-dimensional systems of light and dark bands because they are being viewed from above.  In this picture the system tilted so it can be viewed from the side.

Q:    What is the difference between interference and diffraction?

A:    Both interference and diffraction represent the same physical phenomenon of superposition and combination of waves that meet at the same time in the same point in space.  The extreme results of this superposition are either reinforcement or cancellation of the waves.  The words "interference" and "diffraction" mainly refer to different experimental setups as shown below.

 Number of Slits Name one slit diffraction 2 slits interference N slits diffraction sharp edge diffraction

Q:    How does interference relate to holographic images?

A:    Holographic images are formed as a result of interference between a "reference" light ray and the light ray coming from the same source after having been reflected on an object.

## References

### Facts

Huygens' Principle: all points on a wave front serve as point sources of spherical secondary wavelets.  After a time t, the new position of the wavefront will be that of of a surface tangent to these secondary wavelets.

### Equations

 Wavelength and index of refraction Young's Experiment: bright fringe positions (maxima) d sin = m ,       for  m = 0, 1, 2, . . . Young's Experiment: dark fringe positions (minima) d sin = (m + ) ,     for  m = 0, 1, 2, . . . Intensity in two-slit interference with two waves each of intensity I0 Interference with a thin-film in air: bright (maxima) Interference with a thin-film in air: dark (minima) # List of Topics

 Measurements Electric Potential Magnetism Electrical Circuits (AC) Optical Instruments: Mirrors and Lenses Electrostatics Capacitance Sources of Magnetic Fields Maxwell's Equations Interference Electric Fields Current and Resistance Magnetism in Matter Electromagnetic Waves Diffraction Electric Flux Electrical Circuits (DC) Electromagnetic Induction Interaction of Radiation with Matter: Reflection, Refraction, Polarization  ครั้งที่

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