P4.1 Wave behaviour
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P4.1 Wave behaviour
Mathematical learning outcome:
PM4.1i recall and apply: wave speed (m/s) = frequency (Hz) x wavelength (m)
Assessable content statements:
P4.1a describe wave motion in terms of amplitude, wavelength, frequency and period
P4.1b define wavelength and frequency
P4.1c describe and apply the relationship between wavelength, frequency and the wave velocity
P4.1d apply formulae relating velocity, frequency and wavelength (M1c, M3c)
P4.1e describe differences between transverse and longitudinal waves
P4.1f describe how ripples on water surfaces are used to model transverse waves whilst sound waves in air are longitudinal waves, and how the speed of each may be measured
P4.1g describe evidence that in both cases it is the wave and not the water or air itself that travels
Learners should be familiar with waves from the KS3 topics Light and Sound. Revisiting key language at the start of the topic may be of benefit here: such as wavelength, amplitude, frequency and Hertz (Hz).
Learners should understand that waves transfer energy. Some waves need a medium to travel through, such as sound waves, or longitudinal waves whilst others can travel through a vacuum, such as light waves or transverse waves.
This topic offers lots of opportunities for hands on practical work and demonstrations, which will help to engage learners. A circus of activities could be a great way to revisit KS3 knowledge without taking up too much lesson time.
Diagrams are a useful way for learners to see the differences in amplitude, wavelength and frequency and relate these to the differences in sound they hear. An oscilloscope offers learners this opportunity.
When a wave is produced it disturbs particles. A bigger vibration will disturb particles more than a smaller vibration, and this can be understood by a loud or quiet sound respectively. Amplitude can be defined as the maximum disturbance of a wave from its undisturbed position; the more vibrations made, the louder the sound and therefore the higher the amplitude. Wavelength is the distance between two identical points on a wave, the period is the time taken for one complete wave to pass a certain point, and frequency is the number of waves produced every second, or the number of waves (characterised by one wavelength) that passes a certain point every second.
Wave speed or wave velocity is related to frequency and wavelength via the following equation:
Wave speed = frequency x wavelength
The differences between transverse and longitudinal waves are best seen from diagrams and simulations. Learners should understand that with transverse waves, vibrations are at right angles to the directions of travel and energy transfer. With longitudinal waves the oscillations, or vibrations, travel in the same direction as the energy transfer. Longitudinal waves show areas of compression, where the particles are close together, and areas of rarefaction where particles are spread out. These can be seen in the simulations detailed in the activity section.
Learners also need to understand how waves behave when they travel between two mediums, or reach a surface. Reflection and refraction of light should be familiar through KS3 studies, and this can be used as a basis to move forward at GCSE level.
Reflection of light should have been covered in KS3 but the topic is worth revisiting in order to demonstrate the differences/similarities in light and sound waves.
Sound waves can be reflected from hard, flat surfaces. We sometimes hear this as an echo when the two sounds overlap.
Animals use this to hunt for prey.
Ultrasonic echoes can be used as a measuring tool. If we refer to the equation above, such a high frequency means the wavelength of the sound wave is very small. Sound waves can be reflected back by small objects, echoes can be timed and the distance to the object calculated. Examples of how ultrasonic echoes are used include searching for oil and gas, checking the health and development of foetuses, to check for tumours, investigate heart and liver problems and to break up kidney stones.
When waves arrive at a boundary between two different media at an angle, the velocity changes and causes the wave to change direction. Learners should be familiar with conducting this experiment at KS3. A useful analogy to use here is a car driving on tarmac and then on mud at an angle. The change in material causes the car to slow down and change direction. Sound travels at different speeds in solids, liquids and gases. The wavelength and hence velocity of the sound wave changes: the velocity is directly proportional to the wavelength. Thus if the wavelength doubles, the velocity doubles. Revisiting the equation above may help to demonstrate this relationship.
The human ear
A good way to move on from the frequency range of animals is to compare them to our own hearing range. Again, some knowledge is expected from KS3 studies, and a task whereby learners annotate and describe parts of the ear via a diagram is a great way to elicit what learners already know.
Common misconceptions or difficulties learners may have
Some learners find an increase in scientific terminology hard to grasp, and therefore repetition of and use of key language in this topic is essential.
The direction of travel of a transverse wave is also hard for learners to see. Simulations offer some clarity, but the use of a piece of ribbon or skipping rope can illustrate this idea very well. Conversely, a slinky is a great tool to demonstrate how longitudinal waves travel, and learners can see the clear difference between these two types of wave.
Learners can sometimes confuse reflection and refraction. When discussing changes of velocity in sound waves travelling through two different media, learners may wonder why frequency does not change. The frequency of a sound is determined by the source of the sound, not the medium through which it travels.
Conceptual links to other areas of the specification – useful ways to approach this topic to set learners up for topics later in the course
We revisit transverse waves when we introduce the electromagnetic spectrum so it is important that learners understand the differences between transverse and longitudinal waves, and know the characteristics of transverse waves from the beginning of the topic.
This page has a range of information, mathematical practice questions and tests to introduce you to Waves.
Not all information is relevant but much of it is very useful.
This PowerPoint and exit card is a great idea of how to structure a lesson on refraction.
Some learners may need more specific instructions of how to measure angles, but the idea can be adapted to suit the needs of the class.
An instructional resource with some key teaching notes of how to deliver this in class.
The transverse page comes with a link to a video demonstrating the same phenomena on a wave machine.
Approaches to teaching the content
Light and sound are all around us so this topic lends itself very well to engaging learners with what they can experience around them. Excerpts from learners’ favourite music can be used to test their knowledge about oscilloscope readings and sound characteristics (especially amplitude and pitch).
The behaviour of light at different boundaries can be used to explain our mirror reflections.
Demonstrating how longitudinal waves lose energy – your voice at the front versus back of the class. Q. Why do actors need to project their voices on stage?
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