Post on 02-Jan-2016
description
Today 1/24 HW: 1/23 Handout
“Beats and Boundaries” due Wednesday 1/29
Today:Beats 17.4Reflections at Boundaries 27.3
Monday: Thin Film Interference 27.3 Labs start Monday
Next Week’s Lab: Microwaves A wave in the “electric field” Reflection and transmission from and
through wood Reflection from metal (why metal is bad in
your microwave) Polarization (see section 24.6) Interference (Young’s Double Slit)
Beats
Occur when the frequencies of the sources are not the same
Frequencies must be close Locations for constructive interference move
over time Causes sound to get loud and soft fb “beat frequency” depends on source
frequency difference
Sources emitting different frequencies.
Source 1 Source 2
In this case they are alternately in and out of phase as time goes by.
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
c
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
c
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
c
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
c
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
c
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
c
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
c
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
c
Source 1 Source 2
Sources emitting different frequencies.
In this case they are alternately in and out of phase as time goes by.
c
Now the locations of constructive (and destructive) interference move in time. A stationary listener hears “Beats.”
You want to know the frequency of a tuning fork. You test it by playing it at the same time as a tuning fork with a known frequency of 342 Hz and you hear beats at a rate of 5 per second. You then play it at the same time as one with a known frequency of 349 Hz and the beats are heard at a rate of 12 per second. What is the frequency of the tuning fork?
a. 347 Hz b. 361 Hz c. 345.5 Hz
d. 337 Hz e. 354 Hz.
Fixed End Reflections
Fixed end
Crest turns into trough
Leading edge is the same
See “Wave Interference” handout for how the string looks during the reflection.
Same velocity, length, and amplitude
Free End Reflections
Free end
Crest stays a crest
Leading edge is the same
See “Wave Interference” handout for how the string looks during the reflection.
Same velocity, length, and amplitude
Light to HeavyBoth transmission and reflection
Boundary feels like a fixed end to the light string
Reflection just like fixed end, inverted
Transmitted wavelength has the same shape except it’s shorter in length because it travels slower than the incoming wave.
Slower, so not as far from boundary
Shorter, “bunched up” Inverted wave
Heavy to LightBoth transmission and reflection
Boundary feels like a free end to the heavy string
Reflection just like free end, not inverted
Faster, farther from boundary
Longer, “spread out” Wave not inverted
Transmitted wavelength has the same shape except it’s longer in length because it travels faster than the incoming wave.