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# PH 223 21:6-8

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## Richard Datwyler

on 21 September 2016

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#### Transcript of PH 223 21:6-8

Most interferences is just traveling waves

Standing waves:
they travel and then reflect off the ends
this puts a shift on their PHASE
Speaker problems:
No reflection, we just look at
path differences
phase constant difference
Thin films: these are a mix of the two
They have reflection
but it doesn't always shift their PHASE
Here's the rule: A light moving from n1 - n2
IF n1 < n2 = pi phase shift
(think hitting a heavier object, or fixed wall)
IF n1 > n2 = 0 phase shift
(think less obstruction, free boundary)
A thin film of oil (n=1.25) floats on water (n=1.33). What is the thinnest film that produces a strong reflection for green light with a wavelength of 500 nm?
What is the thinnest film of a material with n = 1.39 on water (n=1.33) that produces a strong reflection for orange light with a wavelength of 600 nm?
Two in phase speaker 2.0 m apart in a plane are emitting 1800 Hz sound waves. Is a point 4.0 m in front of and perpendicular to the speakers plane constructive or deconstructive?
If Sources in PHASE
If exactly out of phase, switch
At this same location, what is the smallest frequency that would produce constructive interference?
outline for the day.

Questions

Finish up 21

Review
"Do nodal and antinodal lines created from 2 sources emanate from a specific epicenter? If so is that epicenter half way between the two sources?"
"So beats is basically two waves with different frequencies that cause both constructive and deconstructive interference? How do differing amplitudes affect this?"
""In following example 21.10 or 11 (depending on your edition) let the speakers be in phase. What is the smallest frequency that will allow destructive interference, keeping all other values the same?""
(2 m, 5 m, 2 m, .73 m, 700 Hz, 341 m/s, .487 m)
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