Waves Flashcards
What are the two type of waves ?
Waves can come in one of two types:
Transverse waves
Longitudinal waves
What are transverse waves ?
Transverse waves are defined as:
Waves that vibrate or oscillate perpendicular to the direction of energy transfer
What do transverse waves do ?
Transverse waves:
oscillate perpendicularly to the direction of travel
transfer energy, but not the particles of the medium
exist as mechanical waves which can travel in solids and on the surfaces of liquids but not through liquids or gases
exist as electromagnetic waves which can move in solids, liquids, gases and in a vacuum
What are examples of transverse waves ?
the highest point above the rest position is called a peak, or crest
the lowest point below the rest position is called a trough
Examples of transverse waves are:
Ripples on the surface of water
Vibrations in a guitar string
S-waves (a type of seismic wave)
Electromagnetic waves (such as radio, light, X-rays etc)
What are longitudinal waves ?
Longitudinal waves are defined as:
Waves where the points along its length vibrate parallel to the direction of energy transfer
What do longitudinal waves do ?
Oscillate in the same direction as the direction of wave travel
Transfer energy, but not the particles of the medium
Move in solids, liquids and gases
Cannot move in a vacuum (since there are no particles)
The key features of a longitudinal wave are where the points are:
Close together, called compressions
Spaced apart, called rarefactions
What are examples of longitudinal waves ?
Examples of longitudinal waves are:
Sound waves
P-waves (a type of seismic wave)
Pressure waves caused by repeated movements in a liquid or gas
Compare transverse & longitudinal waves.
Wave vibrations can be shown on ropes (transverse) and springs (longitudinal)
Waves can be shown through vibrations in ropes or springs
What are the properties of transverse and longitudinal ?
What are waves ?
Waves are disturbances caused by an oscillating source that transfer energy and information without transferring matter
Waves are described as oscillations or vibrations about a fixed point
For example, ripples cause particles of water to oscillate up and down
Sound waves cause particles of air to vibrate back and forth
The wave on the surface of a body of water is a transverse wave
The duck moves perpendicular to the direction of the wave
The duck moves up and down but does not travel with the wave
When describing wave motion , which terms are important ?
When describing wave motion, there are several terms which are important to know, including:
Amplitude (A)
Wavelength (λ)
Frequency (f)
Time Period (T)
What is amplitude defined as ?
Amplitude is defined as:
Amplitude is the maximum or minimum displacement from the undisturbed position
The maximum displacement of a wave is the peak
The minimum displacement of a wave is the trough
Amplitude is measured in metres (m)
What is wavelength defined as ?
Wavelength is defined as
The distance from one point on the wave to the same point on the next wave
In a transverse wave:
The wavelength can be measured from one peak to the next peak
In a longitudinal wave:
The wavelength can be measured from the centre of one compression to the centre of the next
Wavelength is measured in metres (m)
What is the graphical representation of transverse waves?
The amplitude and wavelength of a transverse wave can be represented graphically
The distance along a wave is typically put on the x-axis of a wave diagram
The wavelength is given the symbol λ (lambda) and is measured in metres (m)
The distance along a wave is typically put on the x-axis of a wave diagram
What is frequency defined as ?
requency is defined as:
The number of waves passing a point in a second
Frequency is measured in hertz (Hz)
The unit hertz is equivalent to ‘per second’
5 Hz = 5 waves per second
Waves with a higher frequency transfer a higher amount of energy
What is the time period defined as ?
The time period (or sometimes just ‘period’) of a wave is defined as:
The time taken for a single wave to pass a point
The period is measured in seconds (s)
The equation linking frequency and time period is explained in Frequency & time period
In your exam, you are expected to be able to define these keywords and identify their values from diagrams or scenarios.
The wavelength is often shown graphically between the peaks of two consecutive waves. However, the wavelength can be shown between two corresponding points on two successive waves - the distance will be the same!
What is the wave equation ?
All waves obey the wave speed equation
This is the relationship between the wave speed, frequency and wavelength of a wave
v = f x ^
Where:
v = wave speed in metres per second (m/s)
f = frequency in hertz (Hz)
λ = wavelength in metres (m)
What is the formula triangle for the wave speed equation?
What is frequency and time period ?
Frequency and time period are defined in Describing wave motion
The following equation relates time period and frequency:
f = 1 divided by T
Where:
f = frequency, measured in hertz (Ha)
T = time period, measured in seconds (s)
Visible light has a frequency of about 6 × 1014 Hz.
How long does it take for one complete cycle of visible light to enter our eyes?
Answer:
Step 1: List the known values
Frequency, f = 6 × 1014 Hz
Step 2: State the relationship between frequency and time period
This question involves quantities of time and frequency, so the equation which relates time period and frequency of a wave is:
T = 1/f
Step 3: Substitute the known values into the equation and calculate the time period
T space equals space 1 space divided by space left parenthesis 6 space cross times space 10 to the power of 14 right parenthesis space equals space 1.67 space cross times space 10 to the power of negative 15 end exponent space straight s
Therefore, it takes 1.67 × 10-15 s (to 2 decimal places) for one wave of visible light to pass our eyes
What is the calculations in different contexts?
The wave equation can be applied and rearranged to calculate the properties of Transverse and longitudinal waves
The wave equation applies to all types of waves, including Sound waves and Electromagnetic waves
Exam Tip
When stating equations make sure you use the right letters:
For example, use λ for wavelength, not L or W
If you can’t remember the correct letters, then state the word equations required
Be careful with units: wavelength is usually measured in metres and speed in m/s, but if the wavelength is given in cm you might have to give the speed in cm/s
Likewise, watch out for the frequency given in kHz: 1 kHz = 1000 Hz
What is the Doppler effect is defined as?
The apparent change in observed wavelength and frequency of a wave emitted by a moving source relative to an observer
The Doppler effect can be observed whenever sources of waves move
The frequency of the sound waves emitted by ambulance or police sirens goes from a high pitch (high frequency) to a low pitch (low frequency) as the vehicle whizzes past
Galaxies in outer space emit light waves which appear redder (longer wavelength) to an observer on Earth because the stars are moving away from us
Explain the Doppler Effect ?
Usually, when a stationary object emits waves, the waves spread out symmetrically
To an observer standing in front of an object moving towards them:
The waves appear to get squashed together because the wavelength appears to get shorter (and the frequency appears to get higher)
To an observer standing behind an object moving away from them:
The waves appear to get stretched apart because the wavelength appears to get longer (and the frequency appears to get lower)
Exam Tip
Remember that the Doppler effect is an apparent change in wavelength and frequency. This only happens because a wave emitter moves away from or towards an observer. The speed of the waves emitted stays constant, so if the wavelength of the wave appears to decrease this must mean the frequency appears to increase, and vice versa.
Light is part of a continuous electromagnetic spectrum that consists of which following types of radiation?
radio
microwave
infrared
visible
ultraviolet
x-ray
gamma ray
All waves in the electromagnetic spectrum share the following properties:
They are all transverse
They can all travel through free space (a vacuum)
They all travel at the same speed in free space
What is the EM spectrum ?
The types of radiation found in the electromagnetic spectrum have a specific order based on their wavelength (and frequency)
This order listed above has:
Radio waves at the top because they have the longest wavelength (and highest frequency)
Gamma rays at the bottom because they have the shortest wavelength (and lowest frequency)
Wavelength and frequency are inversely proportional to each other:
An increase in wavelength is a decrease in frequency (towards the red end of the spectrum)
A decrease in wavelength is an increase in frequency (towards the violet end of the spectrum)
This is explained by the Wave equation
What is the visible spectrum ?
Visible light is the only part of the EM spectrum detectable by the human eye
However, it is only a very small part of the whole electromagnetic spectrum
In the natural world, many animals, such as birds, bees and certain fish, can perceive beyond visible light and use infra-red and UV wavelengths of light to see
Each colour within the visible light spectrum corresponds to a narrow band of wavelength and frequency
The different colours of waves correspond to different wavelengths:
Red has the longest wavelength (and the lowest frequency)
Violet has the shortest wavelength (and the highest frequency)
See if you can make up a mnemonic to help you remember the order of the colours of visible light in the EM spectrum!
What are the Uses of EM waves?
What are Radio waves & microwaves?
Both radio waves and microwaves are used in wireless communication
This includes:
Radios
Air traffic communication
Mobile phone communication
At very high intensities microwaves are used to heat things in a microwave oven
What is Infrared?
Infrared is emitted by warm objects and can be detected using special cameras (thermal imaging cameras).
Examples of the uses of infrared are:
Security cameras to see people in the dark
TV remote controls
Transport signals down fibre optic cables
What is visible light ?
Visible light is the only part of the electromagnetic spectrum that the human eye can see
It is also used in fibre optic communication
What is ultraviolet ?
Ultraviolet is responsible for giving you a sun tan, which is your body’s way of protecting itself against the ultraviolet
When certain substances are exposed to ultraviolet, they absorb it and re-emit it as visible light (making them glow)
This process is known as fluorescence
Fluorescence can be used to secretly mark things in special ink, such as banknotes
What are the risks of excessive exposure to EM radiation?
Excessive exposure of the human body to electromagnetic waves can have detrimental effects
Risks from overexposure to certain wavelengths include:
microwaves can cause heat damage to internal organs due to the internal heating of body tissue
infrared can burn the skin
ultraviolet can damage skin cells causing sunburn and blindness
gamma and X-rays can kill cells causing cancer and cell mutations
What are the protective measures against the risks of over-exposure?
Devices using hazardous EM radiation contain safety features that reduce human exposure:
microwaves from microwave ovens are prevented from escaping the oven by the metal walls and metal grid in the glass door
infrared wearing protective clothing such as gloves can prevent the skin from feeling the hear
ultraviolet ray damage to the eyes is reduced by wearing sunglasses that absorb ultraviolet and prevent it from reaching the eyes. Sunscreen also absorbs ultraviolet light preventing it from damaging the skin.
gamma and X-rays damage is reduced through using minimal levels in medicine. Doctors leave the room during x-rays to avoid unnecessary exposure. Radiographers wear radiation badges to measure their level of radiation exposure. People working with gamma rays routinely have their dose levels tested.
Exam Tip
In your exam, you may be asked to explain the hazards of each type of radiation and the safety precautions used to reduce these hazards.
What is refraction and reflection ?
All waves, whether transverse or longitudinal, can be reflected and refracted
Reflection occurs when:
A wave hits a boundary between two media and does not pass through, but instead stays in the original medium
In optics the word medium is used to describe a material that transmits light
Media means more than one medium
When does refraction occur ?
Refraction occurs when:
A wave passes a boundary between two different transparent media and undergoes a change in direction
What is the law of reflection ?
The law of reflection states that:
Angle of incidence (i) = Angle of reflection (r)
Angles are measured between the wave direction (ray) and a line at 90 degrees to the boundary called the normal
The angle of the wave approaching the boundary is called the angle of incidence (i)
The angle of the wave leaving the boundary is called the angle of reflection (r)
What are reflection ray diagrams ?
Reflection ray diagrams
When drawing a ray diagram an arrow is used to show the direction the wave is travelling
An incident ray has an arrow pointing towards the boundary
A reflected ray has an arrow pointing away from the boundary
What are refraction ray diagrams ?
The direction of the incident and refracted rays are also taken from the normal line
The change in direction of the refracted ray depends on the difference in density between the two media:
From less dense to more dense (e.g air to glass), light bends towards the normal
From more dense to less dense (e.g. glass to air), light bends away from the normal
When passing along the normal (perpendicular) the light does not bend at all
The change in direction occurs due to the change in speed when travelling in different substances
When light passes into a denser substance the rays will slow down, hence they bend towards the normal
The only properties that change during refraction are speed and wavelength – the frequency of waves does not change
Different frequencies account for different colours of light (red has a low frequency, whilst blue has a high frequency)
When light refracts, it does not change colour (think of a pencil in a glass of water), therefore, the frequency does not change
Exam Tip
When drawing ray diagrams for reflection:
A simple straight line with an arrow is enough to represent the wave
You do not need to draw the wavefronts unless asked to do so!
Take care to draw the angle correctly
If it is slightly out it won’t be a problem, but if there is an obvious difference between the angle of incidence and the angle of reflection then you will probably lose a mark!
Practice drawing refraction diagrams as much as you can! It’s very important to remember which way the light bends when it crosses a boundary:
As the light enters the block it bends towards the normal line
Remember: Enters Towards
When it leaves the block it bends away from the normal line
Remember: Leaves Away
Don’t forget to draw the arrows for the direction of the light rays and make sure they are drawn with a ruler and a sharp pointed pencil
Core practical 4: investigating refraction
Aim of the experiment
To investigate the refraction of light using transparent rectangular blocks, semi-circular blocks and triangular prisms
To review your understanding of refraction use the revision note Reflection & refraction
Variables
Independent variable = shape of the block
Dependent variable = direction of refraction
Control variables:
Width of the light beam
Same frequency / wavelength of the light
What is the method of this practical ?
- Place the glass block on a sheet of paper, and carefully draw around the rectangular perspex block using a pencil
- Switch on the ray box and direct a beam of light at the side face of the block
- Mark on the paper:
. A point on the ray close to the ray box
. The point where the ray enters the block
. The point where the ray exits the block
. A point on the exit light ray which is a distance of about 5 cm away from the block - Draw a dashed line normal (at right angles) to the outline of the block where the points are
- Remove the block and join the points marked with three straight lines
- Replace the block within its outline and repeat the above process for a ray striking the block at a different angle
- Repeat the procedure for each shape of perspex block (prism and semi-circular)
What are the results ?
Consider the light paths through the different-shaped blocks
The final diagram for each shape will include multiple light ray paths for the different angles of incidences (i) at which the light strikes the blocks
This will help demonstrate how the angle of refraction (r) changes with the angle of incidence
Label these paths clearly with (1) (2) (3) or A, B, C to make these clearer
Use the laws of refraction to analyse these results
You can use the revision note Reflection & refraction to do this
What is the formula triangle for snells law ?
A ray of light enters a glass block of refractive index 1.53 making an angle of 15° with the normal before entering the block.
Calculate the angle it makes with the normal after it enters the glass block.
Core Practical: Investigating Snell’s law
Aims of the experiment
To investigate the refractive index of glass, using a glass block
Variables
Independent variable = angle of incidence, i
Dependent variable = angle of refraction , r
Control variables:
Use of the same perspex block
Width of the light beam
Same frequency / wavelength of the light
What is the method of this practical ?
- Place the glass block on a sheet of paper, and carefully draw around the block using a pencil
- Draw a dashed line normal (at right angles) to the outline of the block
- Use a protractor to measure the angles of incidence to be studied and mark these lines on the paper
- Switch on the ray box and direct a beam of light at the side face of the block at the first angle to be investigated
- Mark on the paper:
. A point on the ray close to the ray box
. The point where the ray enters the block
. The point where the ray exits the block
. A point on the exit light ray which is a distance of about 5 cm away from the block - Remove the block and join the points marked with three straight lines
- Replace the block within its outline and repeat the above process for a rays striking the block at the next angle
What is an example result table ?
What is the analysis of the results ?
If the angles have been measured correctly, the paper should end up looking like this:
Snell’s Law relates the angles of incidence and refraction
This is covered in the Snell’s law revision note
Plot a graph of sin i on the y-axis against sin r on the x-axis
The refractive index is equal to the gradient of the graph
Evaluate the experiment?
Systematic Errors:
An error could occur if the 90° lines are drawn incorrectly
Use a set square to draw perpendicular lines
Random Errors:
The points for the incoming and reflected beam may be inaccurately marked
Use a sharpened pencil and mark in the middle of the beam
The protractor resolution may make it difficult to read the angles accurately
Use a protractor with a higher resolution
Safety considerations
The ray box light could cause burns if touched
Run burns under cold running water for at least five minute
Looking directly into the light may damage the eyes
Avoid looking directly at the light
Stand behind the ray box during the experiment
Keep all liquids away from the electrical equipment and paper
What is total - internal reflection
Sometimes, when light is moving from a denser medium towards a less dense one, instead of being refracted, all of the light is reflected
This phenomenon is called total internal reflection
Total internal reflection (TIR) occurs when:
The angle of incidence is greater than the critical angle and the incident material is denser than the second material
Therefore, the two conditions for total internal reflection are:
The angle of incidence > the critical angle
The incident material is denser than the second material
What are optical fibres ?
Total internal reflection is used to reflect light along optical fibres, meaning they can be used for
communications
endoscopes
decorative lamps
Light travelling down an optical fibre is totally internally reflected each time it hits the edge of the fibre
What is the structure of an endoscope?
How does the Endoscopes utilise total internal reflection to see inside a patient’s body?
What are prisms used for?
Prisms are used in a variety of optical instruments, including
periscopes
binoculars
telescopes
cameras
Prisms are also used in safety reflectors for bicycles and cars, as well as posts marking the edges of roads
A periscope is a device consisting of two right-angled prisms that can be used to see over tall objects
Does the light totally internally reflects in both prisms?
Exam Tip
If asked to name the phenomena make sure you give the whole name – total internal reflection.
Remember: total internal reflection occurs when going from a denser material to a less dense material and ALL of the light is reflected.
If asked to give an example of a use of total internal reflection, first state the name of the object that causes the reflection (e.g. a right-angled prism) and then name the device in which it is used (e.g. a periscope)
What is the critical angle?
As the angle of incidence is increased, the angle of refraction also increases until it gets closer to 90°
When the angle of refraction is exactly 90° the light is refracted along the boundary
At this point, the angle of incidence is known as the critical angle c
A glass cube is held in contact with a liquid and a light ray is directed at a vertical face of the cube. The angle of incidence at the vertical face is 39° and the angle of refraction is 25° as shown in the diagram.
The light ray is totally internally reflected for the first time at X.
Complete the diagram to show the path of the ray beyond X to the air and calculate the critical angle for the glass-liquid boundary.
Opals and diamonds are transparent stones used in jewellery. Jewellers shape the stones so that light is reflected inside. Compare the critical angles of opal and diamond and explain which stone would appear to sparkle more.
The refractive index of opal is about 1.5
The refractive index of diamond is about 2.4
Exam Tip
When calculating the value of the critical angle using the above equation:
First use the refractive index, n, to find sin(c)
Then use the inverse sin function (sin–1) to find the value of c
What wave is light?
Visible light is a part of the Electromagnetic spectrum which means it is a transverse wave
This is explained in Transverse & longitudinal waves
Light can undergo:
Reflection
Refraction
What wave is sound ?
Sound waves are longitudinal waves
This is explained in Transverse & longitudinal waves
Longitudinal waves are usually drawn as several lines to show that the wave is moving parallel to the direction of energy transfer
Drawing the lines closer together represents the compressions
Drawing the lines further apart represents the rarefactions
Sound can also undergo:
Reflection
Refraction
The reflection of a sound wave is called an echo
Core practical 6: investigating the speed of sound
Experiment 1: measuring the speed of sound between two points
he aim of this experiment is to measure the speed of sound in air between two points
Variables
Independent variable = Distance
Dependent variable = Time
Control variables:
Same location to carry out the experiment
- Use the trundle wheel to measure a distance of 100 m between two people
- One of the people should have two wooden blocks, which they will bang together above their head to generate sound waves
- The second person should have a stopwatch which they start when they see the first person banging the blocks together and stop when they hear the sound
- This should be repeated several times and an average taken for the time travelled by the sound waves
- Repeat this experiment for various distances, e.g. 120 m, 140 m, 160 m, 180 m
What are the results ?
Experiment 2: measuring the speed of sound with oscilloscopes
he aim of this experiment is to measure the speed of sound in air between two points using an oscilloscope
Variables
Independent variable = Distance
Dependent variable = Time
Control variables:
Same location to carry out the experiment
Same set of microphones for each trial
- Connect two microphones to an oscilloscope
- Place them about 2 m apart using a tape measure to measure the distance between them
- Set up the oscilloscope so that it triggers when the first microphone detects a sound, and adjust the time base so that the sound arriving at both microphones can be seen on the screen
- Make a large clap using the two wooden blocks next to the first microphone
- Use the oscilloscope to determine the time at which the clap reaches each microphone and the time difference between them
- Repeat this experiment for several distances, e.g. 2 m, 2.5 m, 3 m, 3.5 m
What are the results ?
Analysis of results
The speed of sound can be calculated using the equation:
average speed = distance moved / time taken .
The speed of sound in the air should work out to be about 340 m/s
Evaluating the experiments
Systematic Errors:
In experiment 2, ensure the scale of the time base is accounted for correctly
The scale is likely to be small (e.g. milliseconds) so ensure this is taken into account when calculating speed
Random errors:
The main cause of error in experiment 1 is the measurement of time
Ensure to take repeat readings when timing intervals and calculate an average to keep this error to a minimum
Maximise the distance between the two people where possible. This will reduce the error in measurements of time because the time taken by the sound waves to travel will be greater
Exam Tip
When answering questions about methods to measure waves, the question could ask you to comment on the accuracy of the measurements.
In the case of measuring the speed of sound:
Experiment 2 is the most accurate because the timing is done automatically
Experiment 1 is the least accurate because the time interval is very short
Whilst this may not be too important when giving a method, you should be able to explain why each method is accurate or inaccurate and suggest ways of making them better (use bigger distances)
For example, if a manual stopwatch is being used there could be variation in the time measured which can be up to 0.2 seconds due to a person’s reaction time
The time interval could be as little as 0.3 seconds for sound travelling in the air
This means that the variation due to the stopwatch readings has a big influence on the results and they may not be reliable
Did this page help you?
What is a oscilloscopes ?
An oscilloscope is a device that can be used to study a rapidly changing signal, such as:
A sound wave
An alternating current
How does the sound waveform be displayed on a oscilloscope ?
When a microphone is connected to an oscilloscope, the (longitudinal) sound wave is displayed as though it were a transverse wave on the screen
The properties of longitudinal and transverse waves are explained in the revision note Transverse & longitudinal waves
The time base (like the ‘x-axis’) is used to measure the time period of the wave
The height of the wave (measured from the centre of the screen) is related to the amplitude of the sound
The number of entire waves that appear on the screen is related to the frequency of the wave
If the frequency of the sound wave increases, more waves are displayed on screen
Core practical 7: using an oscilloscope
Aims of the experiment
The aim of this experiment is to investigate the frequency of a sound wave using an oscilloscope
Variables
Independent variable = Tuning forks of different frequencies
Dependent variable = Time period
What is the method of this oscilloscope experiment ?
- Connect the microphone to the oscilloscope as shown in the image above
- Test the microphone displays a signal by humming
- Adjust the time base of the oscilloscope until the signal fits on the screen - ensure that multiple complete waves can be seen
- Strike the tuning fork on the edge of a hard surface to generate sound waves of a pure frequency
- Hold the tuning fork near to the microphone and observe the sound wave on the oscilloscope screen
- Freeze the image on the oscilloscope screen, or take a picture of it
7.Measure and record the time period of the wave signal on the screen by counting the number of divisions for one complete wave cycle - Repeat steps 4-6 for a variety of tuning forks
What are the results of the oscilloscope experiment ?
Analysis of results
To convert the time period of the wave from the number of divisions into seconds, use the scale of the time base. For example:
The time base is usually measured in units of ms/cm (milliseconds per centimetre)
This would mean a wave with a time base of 4 cm has a time period of 4 ms
To calculate the frequency of the sound waves produced by the tuning forks, use the equation:
f = 1/T
Evaluating the experiment
Systematic Errors:
Ensure the scale of the time base is accounted for correctly
The scale is likely to be small (e.g. milliseconds) so ensure this is taken into account when calculating the time period
Random Errors:
A cause of random error in this experiment is noise in the environment, so ensure it is carried out in a quiet location
Exam Tip
You have a lot of core practicals to know about. Make sure you don’t get those relating to sound confused with each other. To succeed in questions about this particular practical you need to know exactly how an oscilloscope works. To do that revise this in the revision note about Sound & oscilloscopes.
What is a pitch sound?
The pitch of a sound is related to the frequency of the vibrating source of sound waves
If the frequency of vibration is high, the sound wave has a high pitch
If the frequency of vibration is low, the sound wave has a low pitch
How do you compare the pitch of sound displayed on an oscilloscope?
What is loudness ?
The loudness of a sound is related to the amplitude of the vibrating source of sound waves
If the sound is loud, the sound wave has a large amplitude
What is the range of human hearing ?
The human ear responds to the vibrations caused by sound waves
The frequency range for human hearing is 20 Hz to 20 000 Hz
Below the frequencies that humans can hear is infrasound
Above the frequencies that humans can hear is ultrasound
Remember that altering the frequency of a sound wave does not affect the volume, only the wave pitch. Changing the amplitude of the wave changes the volume.
