Principle of the Michelson-Morley interferometer.

The idea

The Michelson–Morley interferometer splits a beam of monochromatic light with a part-silvered mirror into two perpendicular paths. Each beam travels to a mirror, reflects, and returns to the same part-silvered mirror, where the beams recombine and fall on a detector or eyepiece. Because the two paths differ slightly in length, the recombining waves have a phase difference, producing bright and dark interference fringes. Rotating the apparatus through 90° swaps the roles of the two arms, so any difference in light travel time along the arms would shift the fringe pattern. Michelson and Morley looked for this shift to detect motion through the aether. No shift was observed, which showed that the speed of light is the same in both arms and independent of the Earth's motion.

What earns the marks

• A part-silvered mirror (beam splitter) divides one light beam into two coherent beams travelling along mutually perpendicular arms.
• Each beam reflects from a mirror at the end of its arm and returns to the part-silvered mirror, where the beams recombine.
• Recombination produces an interference pattern of bright and dark fringes because the two paths have different optical path lengths.
• The path difference determines the phase difference between the recombining waves and therefore the fringe positions.
• Rotating the interferometer by 90° exchanges the two arm lengths relative to any motion through the aether, so a change in travel time would shift the fringes.
• Michelson and Morley observed no measurable fringe shift, which is evidence that the speed of light is invariant and that the aether hypothesis is not supported.

Common mistakes and how to put them right

• Thinking the two beams are incoherent.

  Put it right: The beams originate from the same source and are coherent, which is why stable fringes form.

• Believing the mirrors absorb the light.

  Put it right: The end mirrors reflect the beams back, so each beam traverses its arm twice.

• Assuming the apparatus measures the speed of light directly.

  Put it right: It compares travel times along two arms by observing fringe shifts, not by timing a pulse.

Exam tips

• Sketch the layout with source, part-silvered mirror, two arms, end mirrors and detector, and label the two light paths.
• Explain the role of the 90° rotation explicitly, because this is what converts a possible aether effect into an observable fringe shift.
• Link the absence of a fringe shift to the invariance of the speed of light rather than simply saying the experiment failed.