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Figure 2 shows the apparatus Fizeau used to determine the speed of light - AQA - A-Level Physics - Question 2 - 2017 - Paper 7
Question 2
Figure 2 shows the apparatus Fizeau used to determine the speed of light. The following observations are made. A. When the speed of rotation is low the observer se... show full transcript
Worked Solution & Example Answer:Figure 2 shows the apparatus Fizeau used to determine the speed of light - AQA - A-Level Physics - Question 2 - 2017 - Paper 7
Step 1
Explain these observations.
Answer
Observation A indicates that when the wheel's rotation speed is low, the light has enough time to travel to the mirror and back to the observer; thus, they see the light returning.
In Observation B, the light gets blocked when the speed of the wheel exceeds a certain threshold due to the wheel's teeth obstructing the return path of the light. At this increased speed, the observer cannot see the light because the wheel rotates fast enough that the light pulse does not return to the observer's position before the next tooth blocks it.
Step 2
Discuss whether the data in Table 1 are consistent with the accepted value for the speed of light.
Answer
Using the formula for the speed of light, we have: Calculating this yields: This result is comparable to the accepted value of approximately , confirming the consistency of Fizeau's experiment with the accepted speed of light and showing that both values are similar.
Step 3
Deduce the value of v when the observer would next be unable to see light returning from the mirror.
Answer
As the wheel continues to rotate, the observer will next be unable to see the light returning when the light pulse travels a distance equal to the distance between two adjacent teeth on the wheel. This means the gap must be equal to the distance covered by the light during the time it takes for the wheel to rotate to the next tooth. If the observer sees light until the speed increases to an extent that the time taken for the light to return is longer than the time for the wheel to rotate to the next tooth, then:
.
Step 4
Explain how the nature of light is implied by Maxwell's theory of electromagnetic waves and Fizeau's result.
Answer
Maxwell's theory predicts that light behaves as an electromagnetic wave, which means it propagates through space as oscillating electric and magnetic fields. Fizeau’s experiment measured the speed of light, aligning with Maxwell's predictions, thus confirming that light is not merely a wave in a medium like sound but an electromagnetic wave capable of traveling through the vacuum of space. This unified understanding provides insight into the wave-like properties of light as well as its role in electromagnetic radiation.