PH 105

1
PH 105
Wave Phenomena Reflection, Refraction
Dr. James van Howe Lecture 16
Harvard Physics Light/Optics Demo
2
Name the Artist/Composer

• Ray Charles
• Randy Newman
• Louis Armstrong
• Herbie Hancock

3
Reflection from a Change in Medium
Denser medium slower speed
S.H.O
The velocity goes down on the denser medium
What happens to the wavelength on the denser
medium?

• Gets Shorter
• Gets Longer

4
A sound wave in air hits a pocket of carbon
dioxide at an angle of 30 degrees.
30?
air
denser than air
CO2
What is the angle of the transmitted wave?

• Smaller than 30 degrees (towards the normal)
• Greater than 30 degrees (away from the normal)
• 30 degrees the wave does not bend at all (it
  goes straight through)

5
Turn-in jvanhowian scale exercise debrief
6
Reflection
Demo video, wave on slinky
All waves light, water, sound have property of
reflection off of a boundary
Sound waves
Sound reflects well off of hard, non-porous
materials like plaster, painted concrete, wood
(particularly at high-frequencies)
7
To understand how reflection works, we pretend
the boundary where reflection takes place
(reflector) is a mirror
Demo Rope
Free end
Virtual wave
mirror
constructive interference at free end
Reflected pulse in-phase with incident pulse
8
Reflection at an Angle
Ray of light, or ray of sound at an angle to
reflector
mirror
Virtual Ray
Incident angle
Normal
reflected angle
Normal is drawn perpendicular to boundary
Think about this next time you play pool
9
Reflection
Demo Rope
(special case needs corner mirror)
Fixed end
Virtual wave
Corner mirror
Reflected pulse out of phase with incident pulse
10
Waves in three dimensions
Plane Wave
Spherical wave (point source)
crest
valley
Like dropping a stone in a pond
Looking down on top of sine wave
11
Waves in three dimensions
A small speaker is a point source
Tower of speakers cylindrical wave
Small means speaker lt
12
Reflection of a Spherical Wave
Just before reflection
mirror
A little time later
13
Parabolic reflector
Incoming wave
Can be used to focus sound
Focal Point
500 from spytech.com Claim conversations can be
heard up to 300 yds. away
14
Whispering Gallery
Incoming wave
Listener
Speaker
15
Whispering Gallery accidentally built into St.
Pauls Cathedral in London
16
Reflection from a Change in Medium
Demo video, fat/skinny slinky
17
Reflection from a Change in Medium
Denser medium slower speed
S.H.O
The velocity goes down on the denser medium
What happens to the wavelength on the denser
medium?

• Gets Shorter
• Gets Longer

18
Reflection from a Change in Medium
The wavelength decreases
The frequency doesnt change, but the velocity
decreases since density goes up.
Wavelength, l, must go down to keep frequency the
same.
19
Reflection from a Change in Medium
Incident Pulse
Denser medium slower speed
Part of pulse Reflected
Part of pulse Transmitted
Reflected Pulse Transmitted Pulse Incident
Pulse
We sometimes use the word impedance mismatch to
describe the mismatch between two media in which
waves propagate
20
Refraction bending of a wavefront at a boundary
between two media, due to the change in velocity

• Refraction

Incoming wave
air
water
Slower velocity
21
From Faster to Slower Medium
Incoming sound wave
air
CO2
Higher density
Slower velocity
for entering a medium where velocity
slows down, ray bends towards the normal
Since velocity slows but the frequency is the same
22
Refraction of Water Waves
Shallow areas slow the velocity of water waves
Faster Velocity
Slower Velocity
Shoreline
Water waves parallel to shoreline
23
From Slower to Faster Medium
Incoming sound wave
CO2
air
Faster velocity
for entering a medium where velocity
speeds up, ray bends away from the normal
Since velocity speeds up, but frequency is the
same
24
Normal Incidence
Incoming sound wave
air
CO2
Slower velocity
Normal incidence, no refraction
Wavelength still changes to conserve frequency
25
Helpful Analogy Marching Band
Firm Ground
Muddy Mess
26
Helpful Analogy wheel on an axel
Firm Ground
Muddy Mess
27
Refraction Lens
Video Demo Ripple tank, CO2 Lens
More Curvature, More Bending
No Curvature, No Bending
Faster
Slower
Faster
Plane Wave
Point Source
28
Reflection from a Change in Medium
Less Dense
S.H.O
What happens to frequency on the less dense
medium?

• Gets Shorter
• Gets Longer
• Stays the same

29
Reflection from a Change in Medium
Less Dense
S.H.O
What happens to wavelength on the less dense
medium?

• Gets Shorter
• Gets Longer
• Stays the same

30
A sound wave in air hits a pocket of carbon
dioxide at an angle of 30 degrees.
30?
air
denser than air
CO2
What is the angle of the transmitted wave?

• Smaller than 30 degrees (towards the normal)
• Greater than 30 degrees (away from the normal)
• 30 degrees the wave does not bend at all (it
  goes straight through)

31
A sound wave in air hits a pocket of carbon
dioxide at an angle of 30 degrees.
30?
air
denser than air
CO2
What happens to the wavelength in the CO2?

• It gets smaller
• It gets bigger
• The wavelength doesnt change