Wave Behavior Reflection Refraction Diffraction Reflection Refraction Diffraction Interference...

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

• Rope is connected to a pole.• The last particle is fixed at position and unable

to move.

applet

applet

• When the incident pulse reaches the boundary:

Fixed-end Reflection

Reflected pulse in inverted

A portion of energy is transmitted to the pole

The disturbance returns to the source (left side)

• What happens to the wave characteristics?

1. Speed

2. Wavelength

3. Amplitude

Fixed-end Reflection

Remains the same

Remains the same

Decreases

Free-end Reflection

• Rope is attached to a loosely-fit ring around the pole.

• Last particle is free to move.

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• Reflected pulse is not inverted.

Free-end Reflection

Remains the same

Remains the same

Decreases

Speed

Wavelength

Amplitude

Reflection - Wave diagram

Law of reflection: angle i = angle r

Wavelength remains the same

λ

Wave diagram – wave frontsThese lines represent wave frontsWave 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 TypeReflected 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

Refracted wave direction

Wavelength decreases as wave travels from less dense to denser medium.

Refraction – Beach Erosionanimation

headlandBay

shallowdeep

• 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.

Refraction of Sound Day

Less dense medium

denser medium

Sound waves bends “towards the normal”

• 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 - Day

• 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.

Refraction of Sound - Night

denser medium

Less dense medium

Sound waves bends “away the normal”

• Night time – sound bends downwards.

Refraction of Sound - Night

Loud Thunder?

Cooler air

warmer air

Interference

• two waves meet while travelling along the same medium.

• net effect of the two individual waves.

video

Constructive interference Destructive interference

Constructive Interference

• 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.

applet

Video: wave pool

• 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.

Destructive Interferenceapplet

Video: Microwave interference

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.

ANSConstructive 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.

Quiz 2Constructive Interference:

A, B

Destructive Interference:

C,D,E,F

Standing Wave• 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.

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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

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).

Video: singing glass

Video: unbelievable music

Forced Vibration

• Pluck a guitar string compared to the same string mounted on a guitar.

• What is the difference?

Much louder when string on the guitar is plucked. Why?

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

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.

demo

• Video Demo

ResonanceVideo: motor resonance

Video: pendulum resonance

Video: ghost swing

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

• Plastic tube with air column

Resonance - Experiment

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 /4ג . 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

• 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

Resonance – Sound waves