Lesson Worksheet: Interferometry Physics

In this worksheet, we will practice describing the use of an interferometer to measure very small changes of distance.

Q1:

Light waves were once thought by physicists to require a medium called ether to propagate through. In the year 1887, an experiment was carried out to try and measure the speed of light waves relative to the ether. An interferometer was used. The interferometer had a velocity 𝑣 relative to the ether due to the motion of Earth through the ether. The diagram shows an interferometer where the direction of 𝑣 is perpendicular to the arm AB and parallel to the arm CD of the interferometer. The length of the arms of the interferometer was the same. Which of the following would contribute to the net velocities of light waves relative to the interferometer’s beam splitter if it was assumed that the light waves and the interferometer move relative to a stationary ether?

  • AThe velocity of the beam splitter relative to the ether only
  • BThe velocity of the ether
  • CThe velocities of both the light waves and the beam splitter relative to the ether
  • DThe velocities of the light waves relative to the ether only

Q2:

The diagram shows an interferometer. The mirror in one of the interferometer’s arms moves a distance Δ𝐿. Which color line on the graph of intensity 𝐼 measured at the detector against 𝐿 correctly represents the change of intensity as the mirror moves?

  • ABlue
  • BGreen
  • CRed
  • DNone of these colors

Q3:

The diagram shows an interferometer. The mirror in one of the interferometer’s arms is rotated through a small angle. Which of the following statements about the intensity of the light measured by the detector after the mirror rotates must be true?

  • AThe intensity must decrease.
  • BThe intensity may decrease or stay the same.
  • CThe intensity may increase or stay the same.
  • DThe intensity must increase.
  • EThe intensity may either decrease, increase, or stay the same.

Q4:

The diagram shows an interferometer that uses light waves of wavelength 𝜆. The length of one of the arms of the interferometer decreases by Δ𝐿, which is not drawn to scale. The intensity of the light measured by the detector is the same before and after the interferometer's arm changes length. Which of the following statements about Δ𝐿 must be true?

  • AΔ𝐿 is an integer multiple of 𝜆2.
  • BΔ𝐿 is an odd-numbered integer multiple of 𝜆2.
  • CΔ𝐿 is an even-numbered integer multiple of 𝜆2.
  • DΔ𝐿=𝜆2

Q5:

The diagram shows an interferometer. The lengths of the arms of the interferometer are shown, precise to the nearest nanometer. Light of wavelength 625 nm is emitted and detected. What is the difference in the number of wavelengths of the light emitted that reaches the detector after having traveled along the path through the interferometer that includes the vertical arm and the light emitted that reaches the detector after having traveled along the path through the interferometer that includes the horizontal arm? The beam splitter has a negligible thickness.

  • A2×10
  • B8×10
  • C4×10
  • D8×10
  • E3×10

Q6:

The diagram shows an interferometer that uses light waves of wavelength 655 nm. The length of one of the arms of the interferometer decreases by Δ𝐿, which is not drawn to scale. The graph shows the changes in the intensity measured by the detector while the length of the arm changes. Find Δ𝐿.

This lesson includes 4 additional question variations for subscribers.

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