As you enter Pick up your quiz (sorted by section) 1 Physics 7C SS1 Lecture 3: Interference The reason for studying 2 Agenda
Review practice quiz Review Interference basics Review 1D interference (as studied in DLM 4) Introduce 2D interference 3 Which meets the quiz requirements better? A B C: They are equally good 4 Which meets the quiz requirements better?
A B C: They are equally good 5 What is missing from this response? 6 W 10 A 9.2
V 8.5 E 7 S 6 P 5.2 Z 5
Gives clear instructions that could be carried out. Wavelength, perhaps:measurethe distance from one maximum to the next maximum. This distance is the wavelength. Period, perhaps:usethe string to keepattention focusedon one location. Start time when the water is at a minimum value and stop time when the water next reachesa minimum value. The time recorded is the period. OR record the time it takesfor one peak to travel the distance of a wavelength to get the period. OR Find the frequency by measuring the amount of peaksthat hit a certain point within a given amount of time, li ke 10 seconds, and divide the number counted by the time interval to get the number of cyclesper second Proposedmethod is descriptive, but with a problem. Wavelength: let string out beginning at a peakandstop at the next peak. The string would not actuall y travel with the wave s disturbance. We have modeled the water as bobbing up and down li ke a buoy, and in actuali ty it might move in a circular motion, but it would NOT move with the wave. Proposedmethod is descriptive, but shows a conceptual error. Period: Confusesperiod and frequency. time how long it takes to go
from one peakto the next peakto fi nd frequency. Proposedmethod is too ambiguous to be carried out. Period: usemy stop watch to measureone complete cycle but this instruction doesnt makeit clear when to start/stop timing. Wavelength: you could usethe meter stick to measurethe distance of a wave which would give you wavelength but this instruction doesnt makeit clear how to determine the distance. Does not proposea method that can be carried out. measurepeakto peakfrom the graph but this doesnt work because you cant generate a graph without already knowing the period/wavelength. Determine the wavelength by measuring with the meter stick and the frequency by how many cyclesit takesper second. Like S, but with an error. For instance, indicatesthat wavelength can be found from a y vs. t graph. Does not proposethe method of fi nding period and wavelength to calculate wave speed. 7
Quiz requirement b: method that works for a wave pulse A B C: They are equally good 8 Quiz requirement b: method that works for a wave pulse A B C: They are equally good 9
Q R S T W 10 A 9.2 T 7.7 E 7
R 5 X C 3 0 10 0 0 0 Gives clear instructions that could be carried out. One example: hold the meter stick in place,oriented in the direction the wave moves. Keeptrack of one wave crest asit travels down the stick. Begin time when the crest hits the fi rst end of the stick and stop time
when the crest hits reaches the end. The wave hasthen traveled one meter in the time recorded on the stop watch. The speedin m/s is then (1 m/)(stopwatch time in seconds). Proposedmethod is descriptive, but with a problem. Like in W, but insteadof watching a wave crest travel down the stick plans to let go of the string and time the string s journey from one end of the stick to the other. As with A above, the string would oscill ate in place,not travel with the wave. Proposesmeasuring distance traveled in a time interval, but with a serious error: Often considersthe vertical distanced traveled (the ampli tude) to fi nd the speed tawhich the water s surfacemovesup and down. This is not the same as the wave speed,which is the speedat which the wave propagates. Proposedmethod is too ambiguous to be carried out. Hasideaof seehow many meters the water travels per some time but cannot progressfurther. Does not proposethe method of fi nding how far the disturbance travels in a given time interval. Often will write the wave equation and say something about fi nding the valuesin the equation, but this method will not result in wave speed. OR will proposealternative ways to get period and wavelength, but this wil l not
fi t requirement (b) that one method work for a wavepulse. Only proposes1 method; other method blank Proposesno methods; both methods blank Identifies that wave is 2D States 1D States 3D Other/Blank 10 Interference: When Waves Meet Phenomena:
Interfering pulses on the wave machine Interfering block-wave Using Total Phase Equivalent total phase: Constructive Total phase separated by half-cycle: Destructive 11 Superposition of Waves The period and wavelength are exactly the same. One wave travels to the right, one to the left. 12
Interfering Sound Waves + Greater amplitude Greater pressure fluctuations More & less intense means louder and softer 13 Interfering Sound Waves + No more pressure
fluctuations No sound! Amplitude is zero 14 Phase Chart for Interfering Waves Same rough format, but add difference (most important part!) t 2 T x 2
Unit circle Wave 1 Wave 2 Difference 15 What is responsible for the
destructive interference in this case? a) b) c) d)) e) x=0 x=/2 x None of the above 16 Introducing 2D Interference: The Ripple Tank
Try to spot Places that are very deep Places that are very shallow Places that are normal depth Places two crest combine Places two troughs combine Places a crest meets a trough 17 Two slits create two overlapping wave fronts
d Crest Trough 18 Two slits create two overlapping wave fronts d What type of interference occurs at the marked spot? Crest Trough a)
b) c) d) Constructive Destructive Partial Time-dependent 19 Two slits create two overlapping wave fronts d Why? a) b) c) d)
x=0 x=/2 x None of the above Crest Trough 20 Two slits create two overlapping wave fronts x1 x1=x2 so x=0! x2 Why?
a) b) c) d) x=0 x=/2 x None of the above Crest Trough 21 Two slits create two overlapping wave fronts d What type of
interference occurs at the marked spot? Crest Trough a) b) c) d) Constructive Destructive Partial Time-dependent 22 Two slits create two overlapping wave fronts
d Why? a) b) c) d) x=0 x=/2 x None of the above Crest Trough 23 Two slits create two overlapping wave fronts
d What type of interference occurs at the marked spot? Crest Trough a) b) c) d) Constructive Destructive Partial Time-dependent
24 Two slits create two overlapping wave fronts d Why? a) b) c) d) x=0 x=/2 x None of the above Crest Trough
25 Two slits create two overlapping wave fronts d x1 x1>x2 now x1=2 x2=1.5 x2 Why? a) b) c) d)
x=0 x=/2 x None of the above Crest Trough 26 An interesting phenomenon: What are you seeing? a) b) c) d) e) Constructive Interference Only
Destructive Interference Only Pathlength-dependent Interference: both constructive & destructive depending on position Pathlength-dependent interference: all types (const, dest, and partial) depending on position Partial Interference 27 Laser Interference In DL you will use a shortcut to calculate pathlength difference: d sin = x 28
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