AH 3.3.4 The Superposition of Waves Flashcards
Explain what it means for two waves to be coherent.
Sources of waves are coherent if they have:
a) a constant phase relationship (i.e. are phase-linked)
b) have the same frequency
Explain what is meant by
a) constructive interference (superposition)
b) destructive interference (superposition).
a) In constructive superposition two wave crests or troughs superpose exactly in phase to give a supercrest (maximum) or supertrough (minimum) respectively.
b) In destructive superposition a wave crest superposes exactly with a trough to create cancellation - a resultant wave of zero amplitude..
a) What additional condition is required for two-source interference of visible light to occur?
b) How is this achieved in Young’s double slit interference experiment?
c) Draw a diagram of Young’s arrangement.
a) The two sources must receive their light from a point source (or at least a good approximation to it).
b) Light from an extended source (e.g. a bulb) is passed through a narrow single slit which acts as a point source for the two secondary sources.
c) See the diagram below - So acts as the point source.
To produce a regular interference pattern for waves, what conditions must be met?
There must be two sources of waves which are
a) coherent
b) of the same amplitude
Producing a regular two-source interference pattern for light waves is more difficult than for waves of longer wavelength e.g. microwaves.
a) Why is this?
b) What is done in practice to produce two-source interference with light waves?
a) The wavelength of visible light is so small that an extended source (one with measurable length, breadth and height like a bulb) must be regarded as a very large number of point sources. These point sources emit randomly and so will NOT be coherent.
b) Light from an extended source like a lamp is passed through a narrow single slit which acts as a single point source (or at least, a good approximation to it).
Light from this ‘point source’ may then be passed through two slits producing two secondary sources which are coherent.