Units Used In This Topic:
frequency Hertz (Hz)
magnification No Unit (it is a ratio)
timeperiod second (s)
velocity metre/second (m/s)
wavelength metre (m)
distance metre(m)
speed metre/second (m/s)
time seconds (s)
Waves transfer energy from one plane to another.
Waves can be:
- mechanical: they need a medium to trace through
- electromagnetic: the can travel in a vacuum without a medium
Waves may be either transverse or longitudinal. One whole wave is called an oscillation.
Transverse Waves
The ripples on a water surface are an example of a transverse wave.
The individual particles oscillate perpendicular to the direction of wave energy travel
Longitudinal Waves
The ripples on a water surface are an example of a transverse wave.
Longitudinal waves show areas of compression and rarefaction.
Sound waves travelling through air are longitudinal.
The individual particles oscillate parallel to the direction of wave energy travel
For any mechanical wave, the individual particles that make up the wave do not travel far from their rest position. They just oscillate back and forth by plus or minus the amplitude of the wave.
Question: Describe the difference between transverse waves and longitudinal waves. (2 marks)
in a longitudinal wave, the oscillations / vibrations are parallel to the direction of energy transfer
in a transverse wave, the oscillations / vibrations are perpendicular to the direction of energy transfer
The amplitude of a wave is the maximum displacement of a point on a wave away from its undisturbed position.
The wavelength of a wave is the distance from a point on one wave to the equivalent point on the adjacent wave.
The frequency of a wave is the number of waves passing a point each second. Measured in Hertz, Hz.
The time period of a wave is how many seconds on wave takes to happen. It is 1/frequency.
The wave speed is the speed at which the energy is transferred (or the wave moves) through the medium.
All waves obey the wave equation:
wave speed = frequency × wavelength
Question: A radio wave has:
• a speed of 300 000 000 m/s
• a wavelength of 500 m
Calculate the frequency of the radio wave.
Give the unit.
wave speed = frequency × wavelength
300 000 000 = frequency x 500
frequency = 300 000 000 / 500
The frequency of the wave is 600000 Hertz
Question: An electromagnetic wave has a frequency of 2 400 000 000 Hz
The speed of all electromagnetic waves is 300 000 000 m/s
Calculate the wavelength of the electromagnetic waves.
wave speed = frequency × wavelength
300 000 000 = 2 400 000 000 x wavelength
wavelength = 300 000 000 / 2 400 000 000
The wavelength is 0.125m
Measuring the Speed of Sound Waves Air
Stand approximately 50m from a wall
Make a loud noise, start the timer when you make the noise
Stop the timer when you hear the echo
The speed of the sound is calculated using speed = distance/time
The distance the wave travels is 2 x the distance you are standing from the wall.
Measuring the Speed of Waves on Water
Use a ripple tank screen to observe your waves.
The frequency of the waves being generated is read from the frequency generator.
The wavelength of the waves is found by measuring the length of ‘n’ waves and dividing the by ‘n’ (to improve accuracy)
The wave speed can be found using wave speed = frequency × wavelength
In this example: Frequency = 40Hz
Length of 5 waves = 9cm = 0.09m
Length of 1 wave = 0.09/5 = 0.018 m
wave speed = frequency × wavelength
wave speed = 40 x 0.018 = 0.072 m/s
Question: One student bangs two bricks together.
The sound wave produced is reflected from the wall and travels back to the students.
Describe how they can determine the speed of sound (4 marks)
- measure the distance between the student with the bricks and the wall with a trundle wheel or tape measure
- measure the time taken from banging the bricks to the echo
- double the measured distance to give the distance travelled
- use:
- repeat timings
- remove anomalies
- calculate a mean
Question: Some students want to determine the wave speed of water waves in a ripple tank.
Describe a method the students could use. (4 marks)
- adjust the bar so that it just touches the surface of the water
- adjust motor to produce low frequency wave
- adjust the lamp until the pattern is seen clearly on the card underneath
- place a metre rule at right angles to the waves
- measure the length of a number of waves (minimum 3 waves)
- divide the length by the number of waves to give wavelength
- count the number of waves passing a point in a given time or read the frequency from the frequency generator
- divide the number of waves counted by the time to give the frequency
Refraction
When a wave enters another medium the frequency stays the same but the speed and wavelength change. The wave will change direction because of the change in speed. This is observed as refraction.
If the wave slows down, the wavelength will decrease. If the wave speeds up, the wavelength will increase.
