You're designing a Michelson interferometer in which a speedof-light difference of \(100 \mathrm{m} / \mathrm{s}\) in two perpendicular directions is supposed to shift the interference pattern so a bright fringe of 550 -nm light ends up where the adjacent dark fringe would be in the absence of a speed difference. How long should you make the interferometer's arms?

The interference pattern generated by a Michelson interferometer is a fascinating manifestation of the wave-like nature of light. When two light waves combine, they can interfere with each other, leading to a pattern of alternating bright and dark bands known as 'fringes'. A bright fringe occurs when the waves are in phase, reinforcing each other, while a dark fringe occurs when they are out of phase by half a wave, causing them to cancel out.

To delve deeper into understanding how an interference pattern is formed, imagine two beams of light derived from a single light source split by a beam splitter. After reflecting off mirrors in the interferometer's arms, the beams recombine. Small differences in the path length of the arms result in the beams being slightly out of sync, creating the interference pattern. This pattern is highly sensitive to changes in the optical path length, such as those caused by motion, temperature changes, or, in the context of the exercise, a speed of light difference.

Thus, by observing the shift in the interference pattern—a bright fringe moving to the position of a dark fringe—we can deduce information about the change in optical path difference between the two arms of the interferometer.

In the exercise, we encounter the concept of a 'speed of light difference' which refers to a situation wherein the speed of light appears to differ along two perpendicular paths. This might initially seem puzzling since the speed of light in a vacuum is a constant value, universally recognized as approximately 299,792,458 meters per second. However, in an interferometer, even a hypothetical variation can result in a phase shift that affects the interference pattern.

This slight difference can be due to the apparatus moving relative to a preferred frame or could be theorized in experiments aiming to test the isotropy of the speed of light. In practice, the Michelson interferometer can be used as a precise measuring tool, sensitive to variations to an impressive degree. By adjusting the arm lengths properly, we can calibrate the interferometer to detect or compensate for these very slight differences in speed, indicated by an altered interference pattern, which can be of crucial importance in precision experiments.

Phase difference is the key to translating the changes seen in an interference pattern to measurable quantities. To find the phase difference that corresponds to the shift from a dark to a bright fringe, as required by our exercise, we need to calculate the amount by which the waves' peaks and troughs are misaligned. For a dark fringe, we consider a phase difference of \( (2n+1)\pi \) and for a bright fringe \( 2n\pi \). Thus, the phase shift equates to a difference of \( \pi \), because that is exactly the change needed for the transition between a dark and a bright fringe.

The phase difference is geometrically related to the path difference, with \( \Delta d = \lambda \frac{\phi}{2 \pi} \) where \( \lambda \) is the wavelength of light used and \( \phi \) is the phase difference. Breaking down the phase shift into a mathematical formula allows us not only to determine the specific state of the interference pattern but also to engineer interferometer settings, like arm length, to achieve the desired fringe shifts, giving us precise control over the experimental setup.