AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
(HT only) Students should be able to use wave front diagrams to explain…
Full specification wording
Properties of electromagnetic waves 1: (HT only) Students should be able to use wave front diagrams to explain refraction in terms of the change of speed that happens when a wave travels from one medium to a different medium.
A wave front is a line joining points on a wave that are in step, such as successive crests.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 4 of 5
Study notes
From the checked teaching notes for this specification point.
The idea
A wave front is a line joining points on a wave that are in step, such as successive crests. When plane wave fronts meet a boundary at an angle, one end of each front reaches the new medium before the other. If the wave slows down in the new medium, that end travels more slowly while the other end is still moving faster in the first medium, so the whole front pivots and the direction of travel bends towards the normal. If the wave speeds up, the front pivots the other way and the wave bends away from the normal. The spacing between wave fronts also changes: closer spacing means shorter wavelength and slower speed in that medium. This explains refraction without needing to treat each ray separately.
What earns the marks
- Define a wave front as a line joining points on a wave that are in step, such as adjacent crests.
- Explain that when a wave front meets a boundary at an angle, one part of the front enters the new medium before the rest.
- Link the change in speed in the new medium to the pivoting of the wave front and therefore to the change in direction of the wave.
- State that a wave slowing down bends towards the normal, while a wave speeding up bends away from the normal.
- Relate the spacing of wave fronts to wavelength and speed: fronts closer together indicate a slower wave with shorter wavelength in that medium.
Common mistakes and how to put them right
Thinking that the frequency of the wave changes at the boundary.
Put it right: Frequency stays the same, while speed and wavelength change, so the wave fronts pivot.
Saying that a wave bends because it hits the boundary at an angle, without mentioning the change in speed.
Put it right: The change in speed in the new medium is the cause of the change in direction.
Confusing wave fronts with rays.
Put it right: Wave fronts are lines joining points in step, while rays show the direction of energy transfer and are perpendicular to wave fronts.
Exam tips
- Sketch the boundary and draw at least three wave fronts before and after it, showing the change in spacing and direction.
- Use the phrase 'one end of the wave front reaches the new medium first' to explain the pivoting clearly.
Try the lesson
The start of the Properties of electromagnetic waves 1: (HT only) Students should be able to use wave front diagrams to explain refraction in terms of the change of speed that happens when a wave travels from one medium to a different medium. lesson, made from checked teaching notes for this specification point.
Have a go first
A wave front diagram shows straight wave fronts crossing from deep water into shallow water. In shallow water the wave fronts are closer together. What does this show about the speed of the wave in shallow water?
Listen
A wave front is a line joining points on a wave that are in step, such as successive crests. When plane wave fronts meet a boundary at an angle, one end of each front reaches the new medium before the other. If the wave slows down in the new medium, that end travels more slowly while the other end is still moving faster in the first medium, so the whole front pivots and the direction of travel bends towards the normal. If the wave speeds up, the front pivots the other way and the wave bends away from the normal. The spacing between wave fronts also changes: closer spacing means shorter wavelength and slower speed in that medium. This explains refraction without needing to treat each ray separately. A strong answer earns marks for these points. Define a wave front as a line joining points on a wave that are in step, such as adjacent crests. Explain that when a wave front meets a boundary at an angle, one part of the front enters the new medium before the rest. Link the change in speed in the new medium to the pivoting of the wave front and therefore to the change in direction of the wave. State that a wave slowing down bends towards the normal, while a wave speeding up bends away from the normal. A common slip: Thinking that the frequency of the wave changes at the boundary. Instead, frequency stays the same, while speed and wavelength change, so the wave fronts pivot. Exam tip: Sketch the boundary and draw at least three wave fronts before and after it, showing the change in spacing and direction.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Properties of electromagnetic waves 1: (HT only) Students should be able to use wave front diagrams to explain refraction in terms of the change of speed that happens when a wave travels from one medium to a different medium.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge