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The nature of light questions
For a century the question of whether light is a stream of particles or a wave came down to Newton against Huygens, and Newton's reputation decided it. Young's double slit, Fizeau's measurement of the speed of light and Maxwell's calculation from two electrical constants settled it the other way.
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Give two reasons why Newton's corpuscular theory of light was preferred over Huygens' wave theory for over a century.
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No diffraction of light had been observed, and shadows seemed perfectly sharp, consistent with particles in straight lines (1); and Newton's enormous scientific authority made his theory the default (1).Explain, in general terms, why bright and dark fringes appear in Young's double slit experiment.
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Light spreading from the two slits overlaps on the screen. Where the two sets of waves arrive in step they reinforce, giving a bright fringe (1); where they arrive out of step they cancel, giving darkness (1).State what the constants ε0 and μ0 each describe.
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ε0, the permittivity of free space, relates to the electric field strength due to a charged object in free space (1); μ0, the permeability of free space, relates to the magnetic flux density due to a current-carrying wire (1).State two behaviours of light that Newton's corpuscular theory and Huygens' wave theory could both explain.
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Reflection (1) and refraction (1): both theories accounted for these, disagreeing only over the speed of light in the denser medium.State the nature of an electromagnetic wave.
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Oscillating electric and magnetic fields at right angles to each other and to the direction of travel (1); the wave is self-propagating and needs no medium (1).Calculate the speed of electromagnetic waves predicted by Maxwell's formula c = 1/√(μ0ε0), using μ0 = 4π × 10−7 H m−1 and ε0 = 8.85 × 10−12 F m−1, and state the significance of the result.
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c = 1/√(μ0ε0) (1)
c = 3.00 × 108 m s−1 (1)
Equal to the measured speed of light, implying light is an electromagnetic wave (1)Hertz set up stationary radio waves with adjacent nodes 1.65 m apart, using an oscillator of frequency 9.1 × 107 Hz. Calculate the wave speed and state what the result demonstrated.
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Adjacent nodes are half a wavelength apart, so λ = 3.3 m (1)
c = fλ = 3.0 × 108 m s−1 (1)
Radio waves travel at the speed of light, confirming Maxwell's prediction (1)In Fizeau's experiment a wheel with 720 teeth first blocked the returning light at 12.6 revolutions per second, with the mirror 8.63 km away. Calculate the speed of light this gives.
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First blocking: the wheel advances half a tooth, 1/1440 of a turn, during the round trip (1)
t = 1/(1440 × 12.6) = 5.51 × 10−5 s (1)
c = 2d/t = 2 × 8630/(5.51 × 10−5) (1)
c = 3.1 × 108 m s−1 (1)Explain why Young's fringes cannot be accounted for by a particle theory of light.
In a double slit experiment, light from the two slits arrives at a point on the screen with a path difference of 2.5 wavelengths. State and explain what is observed at this point.
The speed of light in free space is 3.00 × 108 m s−1 and μ0 = 4π × 10−7 H m−1. Use Maxwell's formula c = 1/√(μ0ε0) to determine a value for ε0, the permittivity of free space.
Measurements descended from Fizeau's method later compared the speed of light in air and in water. Explain why this comparison was decisive between the two theories of light.
Explain the significance of Hertz's experiments for the understanding of the electromagnetic spectrum.
Young published his double slit results in 1801, yet the wave theory was not generally accepted for decades. Suggest why.
A physics society plans to repeat Fizeau's experiment with a wheel of 600 teeth and a mirror on a hillside 9.5 km away. Their motor can spin the wheel at up to 15 revolutions per second. Deduce whether they will be able to observe the first blocking of the returning light. c = 3.0 × 108 m s−1.
In Fizeau's experiment the wheel's rotation rate is slowly increased beyond the first blocking, up to twice that rate. Explain what is seen at the eyepiece as this happens.
Light does in fact diffract, yet for over a century nobody observed it doing so and shadows appeared perfectly sharp. Suggest why the diffraction of light escaped detection, and explain why sharp shadows appeared to support Newton.
Describe Fizeau's determination of the speed of light, and explain how the speed is calculated from the measurements. State one reason the measurement was significant.
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