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Chapter28[1]

Chapter28[1] - Chapter 28 SUPERPOSITION AND INTERFERENCE...

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Chapter 28 SUPERPOSITION AND INTERFERENCE: For (b), which peak is which? They all look the same. 1. Addition of ordinates – when adding two waves their y-coordinates (amplitudes) add together at every point to produce a new wave. 2. IN PHASE means that the peaks and troughs of the two waves “line up” with each other. 3. When two waves combine in phase, the resulting wave is the maximum possible. This is called CONSTRUCTIVE INTERFERENCE. For light waves, this would be a bright spot or line. 4. When two waves are 90° out of phase (peak to trough) the result is a null wave, or no wave at all. This is called DESTRUCTIVE INTERFERENCE . This would appear as a dark spot or line. The above rules apply to light that is both MONOCHROMATIC (same color/frequency) and COHERENT (the phase different between the two waves remains constant). To determine whether there will be constructive or destructive interference at some point, it’s necessary to find the phase difference (where is the peak of one wave compared to the other wave?)between the two waves at that point. 1. Find the length difference that the two waves travel, called the path length difference.

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2. If this length is a multiple of whole wavelenths, we get constructive interference. 3. If this length is an odd half-wavelength, we get destructive interference. 2 1 1 2 1 2 m constructive interference (m ) destructive interference 4. •• A person driving at 17 m/s crosses the line connecting two radio transmitters at right angles, as shown in Figure 28–31. The transmitters emit identical signals in phase with each other, which the driver receives on the car radio. When the car is at point A the radio picks up a maximum net signal. (a) What is the longest possible wavelength of the radio waves? (b) How long after the car passes point A does the radio experience a minimum in the net signal? Assume that the wavelength has the value found in part (a). (a) (450m 150m) 300m m is maximum when m is minimum  
1 2 2 2 2 1 2 1 2 2 2 2 1 1 2 2 y vt y t We want y such that (m ) v y (450m) y (150m) 2 ( ) 2( ) ( ) 4   2 YOUNG’S DOUBLE-SLIT EXPERIMENT: (1773-1829) – helped to decipher the Rosetta Stone.

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