Waves

• A wave is a means of transferring energy.

A Transverse WaveThe wave is vibrating perpendicular to the direction that the energy is moving. E.g. Light.

Polarisation

A polarised wave vibrates in one plane only.The fact that light can be polarised proves that it

is a transverse wave.

Stress Polarisation

Proof that light is a transverse wave

Pass light through two pieces of polaroid. Rotate one of them through 900. You will not be able to see through as light is polarised as it passes through the first piece of polaroid and cannot pass through the second. This proves that light is transverse.

NotePolarised light vibrates in one direction only.

A longitudinal waveThe wave vibrates in the same direction that the energy is moving. E.g. sound

DiffractionDiffraction is the spreading of a wave when passing an obstacle or through a gap.

The closer the size of the gap to the wavelength of the wave, the greater the diffraction effect.

Interference

Interference causes the change in amplitude that occurs when waves meet.

Interference

Constructive interference : The waves are in phase and combine to produce a greater amplitude.

Destructive interference: The waves are 1800 out of phase and cancel each other out.

Homework

1. Define and give an example of (i) a transverse and (ii) a longitudinal wave.

2. A wave source emits a wave of wave length 1.5m with a frequency of 5Hz. What is the speed of the waves?

3. What is meant by diffraction? How is it made more pronounced?

4. What is meant by interference?

Coherent waves – In phase and same frequency and amplitude

Non coherent waves – Out of phase and destructive interference will occur.

The Ripple Tank

The Doppler effect

The apparent change in the frequency of a wave due to the motion of the wave source.The frequency increases as the source of sound is moving towards the observer and decreases as the source moves away.

To demonstrate the Doppler effect in the lab

Apparatus: buzzer and batteriesMethod: Place the buzzer and battery attached into a sock.Note the sound of the buzzer.Swing the buzzer in a horizontal circle and note the change in frequency. This is the Doppler Effect

The Doppler Effect

Harmonics in a stretched stringGuitar, cello, violin

f

2f

3f

Fundamental frequency f makes the longest wave that can fit, while other waves that fit are overtones .The fundamental frequency is the lowest frequency of vibration for that object.

All harmonics are present

Harmonics in a closed pipee.g. Clarinet and trumpet

Only odd harmonics are present

f 3f 5f

Harmonics in an open pipee.g. flute

All harmonics are present.F 2F 3F 4F 5F

SOUND

•Sound is a wave produced by vibrations.•An echo is a reflected sound.•Sound travels slower than light.•Sound needs a medium to travel in. There is no sound in a vacuum.•To prove that sound is a wave we must show that it undergoes interference and diffraction.

To prove that sound is a waveMethod: Strike a tuning

fork and rotate it beside

your ear.

Result: The sound

increases and decreases

Reason: Sound

vibrations are undergoing

interference.

Speed of sound in various media

• The more dense the medium the greater the speed.

• The speed of sound in air varies with temperature.

• The greater the temperature of the air the faster sound travels.

Sound needs a medium to travel in.

As air is removed from the bell jar, the sound of the bell reduces until it can no longer be heard, even though the hammer is still hitting the gong.

Factors affecting soundFrequency of a sound wave controls pitch.

Amplitude of a sound wave controls loudness.

Quality is affected by the number of overtones present.

Frequency limits of Audibility

• These are the highest and lowest audible frequencies for the human ear.

20Hz to 20kHz

Dog whistle

Resonance

• This is the transfer of energy between objects of the same natural frequency.

• Tacoma Bridge• Bartons Pendulums-Lab

demonstration of resonance• Tuning forks

Beats

• Beats are created when sound waves of slightly differing frequency meet.

Sound intensity level

• Sound intensity is the rate at which sound energy passes unit area at right angles to the direction in which the sound is travelling.

• The unit: W/m2

• Sound intensity level is measured in decibels.• Threshold of hearing is the lowest audible sound

intensity at a frequency of 1000Hz. (10-12 W/m2)• When the sound intensity doubles the sound

intensity level goes up 3dB.

Harmonics in a stretched stringGuitar, cello, violin

f

2f

3f

Fundamental frequency f makes the longest wave that can fit, while other waves that fit are overtones .The fundamental frequency is the lowest frequency of vibration for that object.

All harmonics are present

Harmonics in a closed pipee.g. Clarinet and trumpet

Only odd harmonics are present

f 3f 5f

Harmonics in an open pipee.g. flute

All harmonics are present.

OvertonesOvertones are the other frequencies besides the fundamental that exist in musical instruments. Instruments of different shapes produce different overtones.

For example, both the waves are the same frequency, and therefore the same note. But their overtones are different, and therefore their sounds are different.