Wave Behavior
• Reflection
• Refraction
• Diffraction
• Interference
• Doppler Effect
Boundary Behavior
• A sound wave travelling through water reflects off the
submarine and returns to its original source.
• Does reflection of a wave affect the speed of the wave?
• The behavior of a wave (or pulse) upon reaching the end of a
medium is referred to as boundary behavior.
Fixed-end Reflection
applet
applet
• Rope is connected to a pole.
• The last particle is fixed at position and unable
to move.
Fixed-end Reflection
• When the incident pulse reaches the boundary:
Reflected pulse in inverted
A portion of energy is transmitted to the pole
The disturbance returns to the source (left side)
Fixed-end Reflection
• What happens to the wave characteristics?
1. Speed
Remains the same
2. Wavelength
Remains the same
3. Amplitude
Decreases
Free-end Reflection
applet
• Rope is attached to a loosely-fit ring around
the pole.
• Last particle is free to move.
Free-end Reflection
• Reflected pulse is not inverted.
Speed
Remains the same
Wavelength
Remains the same
Amplitude
Decreases
Reflection - Wave diagram
Wavelength remains the same
λ
Law of reflection: angle i = angle r
Wave diagram – wave fronts
These lines represent wave fronts
Wave fronts is a imaginary line that joins up particles of the
same phase together.
Direction of wave. It is perpendicular to the wave fronts.
Wave diagram – wave fronts
All the particles along the wave fronts are in phase.
All are “crests” of a transverse wave
Refraction
• Consider transmission of the rope wave from less
dense medium ( the thin rope) towards the boundary
with a more dense medium (the thick rope).
Refraction
wavelength
frequency
speed
Type
Reflected wave
same
same
same
inverted
Transmitted wave
smaller
same
slower
Same as source
Refraction – wave diagram
applet
Video: ripple tank
Water waves travelling from less
dense to denser medium:
Speed decreases
Frequency remains the same
wavelength decreases
Refraction – wave diagram
Incident wave direction
Wavelength decreases as
wave travels from less dense
to denser medium.
Refracted
wave direction
Refraction – Beach Erosion
headland
shallow
Bay
deep
animation
Refraction of Sound Day
Sound waves bends
“towards the normal”
• During the day the air is
warmest near the ground
and cooler away from
ground.
• Sound wave closest to
the ground is fastest, and
the wave farthest above
the ground is travelling
the slowest.
denser medium
Less dense
medium
Refraction of Sound - Day
• Day time – sound bends upwards.
• A "shadow zone" region created in which sound wave cannot
penetrate.
• Person standing in the shadow zone will not hear the sound
even though he/she might be able to see the source
Refraction of Sound - Night
Sound waves bends
“away the normal”
• During the night the air is
cooler near the ground
and warmer away from
ground.
• Sound wave closest to
the ground is slowest,
and the wave farthest
above the ground is
travelling the fastest.
Less dense medium
denser medium
Refraction of Sound - Night
• Night time – sound bends downwards.
warmer air
Cooler air
Loud Thunder?
Interference
video
• two waves meet while travelling along the
same medium.
• net effect of the two individual waves.
Constructive interference
Destructive interference
Constructive Interference
applet
Video:
wave pool
• occurs at any location along the medium where the two
interfering waves have a displacement in the same direction.
• as a result, the medium has a resultant displacement which is
greater than the displacement of the two interfering pulses.
Destructive Interference
applet
Video: Microwave interference
• occurs at any location along the medium where the two
interfering waves have a displacement in the opposite
direction.
• resultant displacement is either zero or smaller than the
original displacement of both waves.
After Interference?
• Interestingly, the meeting of two waves along a medium does
not alter the individual waves or even deviate them from their
path.
• two waves will meet, produce a resultant shape of the
medium, and then continue on doing what they were doing
before the interference.
Quiz 1
• Several positions along the medium are labeled with a letter.
Categorize each labeled position along the medium as being a
position where either constructive or destructive interference
occurs.
ANS
Constructive Interference:
G, J, M , N
Destructive Interference:
H,I,K,L,O
Quiz 2
• Jimmy and Johnny are both creating a series of circular waves
by jiggling their legs in the water. The waves undergo
interference and create the pattern represented in the
diagram at the right. The thick lines in the diagram represent
wave crests and the thin lines represent wave troughs.
Several of positions in the water are labeled with a letter.
• Categorize each labeled position as being a position where
either constructive or destructive interference occurs.
Constructive Interference:
A, B
Quiz 2
Destructive Interference:
C,D,E,F
Standing Wave
applet
• A standing wave pattern results from the
interference of two or more waves along the same
medium.
• All standing wave patterns are characterized by
nodes.
Standing Wave - Nodes
• Nodes occur when two waves interfere such that one wave is
displaced upward the same amount that a second wave is
displaced downward.
• Destructive interference leads to a point of "no displacement."
A node is a point of no displacement.
Standing Wave - Antinodes
• There are other points along the medium which undergo
vibrations between a large positive and and large negative
displacement.
• These are the points which undergo the maximum
displacement during each vibrational cycle of the standing
wave. In a sense, these points are the opposite of nodes, and
so they are called antinodes.
Standing Wave
• Standing wave can only be obtained when the proper frequency is used,
such that the interference of the incident wave and the reflected wave
produces specific points along the medium which appear to be standing
still nodes and the antinodes
• nodes and antinodes are not actually part of a wave. Recall that a
standing wave is not actually a wave but rather a pattern which results
from the interference of two or more waves.
Natural Frequency
• Nearly all objects, when hit or struck or plucked or strummed
or somehow disturbed, will vibrate. If you drop a stick or
pencil on the floor, it will begin to vibrate.
• The frequency or frequencies at which an object tends to
vibrate when disturbed is the natural frequency of the object.
Natural Frequency
Video:
singing glass
Video: unbelievable music
The actual frequency is dependent upon :
1. the properties of the material the object is made of
(this affects the speed of the wave)
2. length of the material
(this effects the wavelength of the wave).
Forced Vibration
• Pluck a guitar string compared to the same
louder when string
string mounted on a guitar. Much
on the guitar is plucked.
Why?
• What is the difference?
Forced Vibration
• When the string is attached to the sound box of the guitar,
the vibrating string forces the sound box to vibrate at its
natural frequency.
• The sound box in turn forces air particles inside the box to
vibrate at the same natural frequency as the string.
Sound box
demo
Forced Vibration
• The entire system (string, guitar, and enclosed air) vibrates
and forces surrounding air particles into vibrational motion.
• The tendency of one object to force an adjoining object into
vibrational motion is referred to as a forced vibration.
Resonance
Video: motor resonance
Video: pendulum resonance
Video: ghost swing
• Video Demo
Resonance
• Resonance occurs when one object vibrating at the
same natural frequency of a second object forces
that second object into vibrational motion.
• Result of resonance is always a very large vibration.
Tacoma Narrows bridge
Washington, 1940
video
Resonance - Experiment
• Plastic tube with air column
When the natural frequency of
the air column is tuned to the
frequency of the vibrating tuning
fork, resonance occurs and a
loud sound results.
Resonance occurs at odd
multiples of ‫ג‬/4. Why?
Resonance – Standing wave
• When an object is forced into resonance vibrations at one of
its natural frequencies, it vibrates in a manner such that a
standing wave is formed within the object.
• Such patterns are only created within the medium at specific
frequencies of vibration. These frequencies are known as
harmonic frequencies or merely harmonics.
Resonance – Sound waves
• Demo - Chladni plate, violin bow and salt
Resonance – Sound waves
• Pattern formed is the standing wave pattern
associated with one of the natural frequencies of
the Chladni plate.
• Salt vibrate and tumble about the plate to reaches
point along the plate which are not vibrating.(nodes)
Resonance – Sound waves
• Music and Harmonics