WAVE SUPERPOSITION

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John Parkinson St. Brendan’s College 1 WAVES John Parkinson St. Brendan’s Sixth Form College

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John Parkinson St. Brendan’s College 2 ADD THEM !!! If two or more travelling waves are moving through some medium, the resultant wave displacement at any point is the algebraic sum of the individual wave displacements. THE PRINCIPLE OF SUPERPOSITION

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John Parkinson St. Brendan’s College 3 + =

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John Parkinson St. Brendan’s College 4 The combination of separate waves in the same region of space to produce a resultant wave is called INTERFERENCE e.g. between two dippers in a Ripple Tank DIPPERS

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John Parkinson St. Brendan’s College 5 + These two waves arrive IN PHASE CONSTRUCTIVE INTERFERENCE HOW DO THEY ADD UP? This is called?

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John Parkinson St. Brendan’s College 6 These two waves arrive in ANTI-PHASE HOW DO THEY ADD UP? DESTRUCTIVE INTERFERENCE This is called? +

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John Parkinson St. Brendan’s College 7 CONDITIONS FOR A PERMANENT INTERFERENCE PATTERN The sources must be coherent, i.e. they must be in phase with one another or they must maintain a constant phase relationship. The sources must have the same wavelengths. The sources must have similar amplitudes. The sources must have the same plane of polarisation.

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John Parkinson St. Brendan’s College 8 S1 and S2 are two coherent sources All points on a wavefront are in phase with one another Along the nodal lines, destructive interference occurs. Here antiphase wavefronts meet.

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John Parkinson St. Brendan’s College 9 double slit screen Monochromatic light, wavelength l Young’s Double Slits A series of dark and bright fringes on the screen.

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John Parkinson St. Brendan’s College 10 Young’s Double Slits THIS RELIES INITIALLY ON LIGHT DIFFRACTING THROUGH EACH SLIT. Where the diffracted light overlaps, interference occurs INTERFERENCE Diffraction Some fringes may be missing where there is a minimum in the diffraction pattern

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John Parkinson St. Brendan’s College 11 Wave trains AP & BP have travelled the same distance (same number of l’s) Assuming the sources are coherent Hence waves arrive in-phase CONSTRUCTIVE INTERFERENCE (Bright fringe)

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John Parkinson St. Brendan’s College 12 When S2P - S1P = (1/2) of l, the waves arrive at P in antiphase to produce a minimum or a dark fringe What happens at P? The general condition for a minimum is: Where n = 0, 1, 2, 3 ...

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John Parkinson St. Brendan’s College 13 When S2P - S1P = l, the waves arrive at P in phase to produce a maximum intensity or a bright fringe What happens at P? The general condition for a maximum is: Where n = 0, 1, 2, 3 ...

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John Parkinson St. Brendan’s College 14 d s s = slit separation w = fringe separation

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John Parkinson St. Brendan’s College 15 Normal light sources emit photons at random, so they are not coherent. LASERS EMIT COHERENT LIGHT