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Physics214Week13

# Physics214Week13 - This Week Waves Standing waves Musical...

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This Week Waves Standing waves Musical instruments, guitars, pianos, organs Interference of two waves tuning a piano, color of oil films Polarisation Why have polaroid sun glasses? Electromagnetic waves and telescopes How do we see color 10/13/2011 Physics 214 Fall 2011 1

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10/13/2011 Physics 214 Fall 2011 2 Periodic waves One can propagate waves which are a single complicated pulse e.g. an explosion or a complicated continuous wave e.g. the wind. We will focus on regular repetitive waves These waves have a pattern which repeats and the length of one pattern is called the wavelength λ The number of patterns which pass a point/second is called the frequency f and if the time for one pattern to pass is T then f = 1/T v = λ /T = f λ λ
10/13/2011 Physics 214 Fall 2011 3 Waves on a string If we shake the end of a rope we can send a wave along the rope. The rope must be under tension in order for the wave to propagate v = (F/ μ ) F = TENSION μ = MASS/UNIT LENGTH

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10/13/2011 Physics 214 Fall 2011 4 Standing waves If two identical waves exist on the same string but traveling in opposite directions the result can be standing waves in which some points never have a deflection, these are called nodes Some points oscillate between plus and minus the maximum amplitude, these are called antinodes. Standing waves provide the notes on musical instruments. When a string is secured at both ends and plucked or hit the generated waves will travel along the string and be reflected and set up standing waves.
10/13/2011 Physics 214 Fall 2011 5 Musical notes Each end of the string must be a node so the possible standing waves must be multiples of λ /2 Fundamental f = v/ λ = v/2L 2 nd Harmonic f = v/ λ = v/L 3 rd Harmonic f = v/ λ = 3v/2L Musical sound is a mixture of harmonics modified by the body of the instrument. v = (F/ μ ) so a piano or a violin is tuned by changing the tension in the string

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