AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Fizeau’s determination of the speed of light and its implications.
Full specification wording
Electromagnetic waves (A-level only): Fizeau’s determination of the speed of light and its implications.
Fizeau measured the speed of light using a rotating toothed wheel and a distant mirror.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Fizeau measured the speed of light using a rotating toothed wheel and a distant mirror. Light passes through a gap, travels to the mirror and returns; if the wheel rotates by half a tooth spacing during the round trip, the returning light is blocked by the next tooth. Knowing the wheel's rotation rate, tooth count and the mirror distance allows calculation of $c$. The method showed that light travels at a finite, measurable speed. Crucially, Fizeau's value for $c$ was very close to the speed of electromagnetic waves predicted by Maxwell's theory. This provided strong evidence that light is an electromagnetic wave.
What earns the marks
- Describes the apparatus: a rotating toothed wheel, a distant mirror and a light source, with light passing through a gap between teeth.
- Explains the blocking condition: the wheel rotates by half a tooth spacing during the light's round trip, so returning light meets a tooth.
- Uses the relationship between wheel rotation rate, number of teeth and time of flight to calculate the speed of light.
- States that the result showed light has a finite speed, contradicting instantaneous propagation.
- Discusses the implication that Fizeau's measured speed of light closely matched Maxwell's predicted speed of electromagnetic waves, supporting the theory that light is an EM wave.
Common mistakes and how to put them right
Confusing the blocking condition with a full tooth spacing.
Put it right: The wheel rotates by half a tooth spacing during the round trip for the first eclipse.
Assuming the light travels only to the mirror rather than to the mirror and back.
Put it right: Use twice the mirror distance in the calculation.
Stating that a finite speed alone proved the wave model.
Put it right: Explain that it was the numerical match with Maxwell's predicted speed of EM waves that supported the electromagnetic nature of light.
Exam tips
- Sketch the apparatus and mark the path of light to make the geometry clear.
- State the blocking condition explicitly before substituting values into any calculation.
- Link the numerical result to Maxwell's theory: state that Fizeau's $c$ matched Maxwell's predicted speed of EM waves.
Try the lesson
The start of the Electromagnetic waves (A-level only): Fizeau’s determination of the speed of light and its implications. lesson, made from checked teaching notes for this specification point.
Have a go first
In a Fizeau-type experiment, light passes through a gap between the teeth of a rotating wheel, travels to a distant mirror and returns. The wheel has 720 teeth and rotates at 20.0 revolutions per second. The returning light is first blocked when the wheel has turned so that a tooth now occupies the position where the gap was. The mirror is 6.0 km away. Which value of the speed of light follows from these measurements?
Listen
Fizeau measured the speed of light using a rotating toothed wheel and a distant mirror. Light passes through a gap, travels to the mirror and returns; if the wheel rotates by half a tooth spacing during the round trip, the returning light is blocked by the next tooth. Knowing the wheel's rotation rate, tooth count and the mirror distance allows calculation of c. The method showed that light travels at a finite, measurable speed. Crucially, Fizeau's value for c was very close to the speed of electromagnetic waves predicted by Maxwell's theory. This provided strong evidence that light is an electromagnetic wave. A strong answer earns marks for these points. Describes the apparatus: a rotating toothed wheel, a distant mirror and a light source, with light passing through a gap between teeth. Explains the blocking condition: the wheel rotates by half a tooth spacing during the light's round trip, so returning light meets a tooth. Uses the relationship between wheel rotation rate, number of teeth and time of flight to calculate the speed of light. States that the result showed light has a finite speed, contradicting instantaneous propagation. A common slip: Confusing the blocking condition with a full tooth spacing. Instead, the wheel rotates by half a tooth spacing during the round trip for the first eclipse. Exam tip: Sketch the apparatus and mark the path of light to make the geometry clear.
Key words
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Blocking condition
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Maxwell's theory
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Finite speed
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Electromagnetic wave
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See every definition
- Blocking condition
- The wheel rotates by half a tooth spacing during the light's round trip, so returning light meets a tooth.
- Maxwell's theory
- Predicts the speed of electromagnetic waves, and Fizeau's value for c was very close to this predicted speed.
- Finite speed
- The result showed light has a finite speed, contradicting instantaneous propagation.
- Electromagnetic wave
- Fizeau's value for c closely matching Maxwell's predicted speed provided strong evidence that light is an electromagnetic wave.
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