Interferometer
An interferometer is a device constructed to split a beam of light into two perpendicular beams and then to bring the two beams together again. Any difference between the beams creates a pattern of interference that manifests itself as a group of bright spectral lines called fringes. Originally designed to measure the fluctuations in the velocity of light, interferometers are now widely used in spectroscopy, chemistry, and precision metrology and inspection.
In 1881, Albert Michelson (1852-1931) was conducting experiments to determine the speed of light. Trained in classical physics, Michelson was a firm believer in the existence of an aether-wind; at that time, light was imagined as an undulating wave, like ripples on a body of water. Just as those ripples need water to move through, light also required a medium for travel, which scientists at the time called the aether.
The intent of Michelson's experiment was to detect the drifting aether by sending the interferometer's perpendicular beams into it--one beam across the current, one beam against it. Michelson expected that the beam moving against the current would be slowed down by the aether wind, and his interferometer would indicate the difference in velocities by displaying an interference pattern. Much to his surprise and dismay, the results of his research were null: no evidence of an aether drift could be found.
Though he repeated his experiment with Edward Morley (1838-1923) in 1878 with the same outcome, Michelson never produced conclusive proof of an aether--in fact, the Michelson-Morley Experiment served as an epitaph for the old ether theory and paved the way for the new optical theories that ultimately led to Albert Einstein's (1879-1955) special theory of relativity in 1905.
Since its now-infamous debut, the interferometer has undergone a number of modifications and has been specialized for use in a variety of fields. In 1893, Michelson used it to measure the International Prototype Meter in Paris in units of wavelength (at that time, this metal bar was the standard for measuring length). Michelson's measurement set a new international standard3/4the use of the unvarying wavelength to measure length. To this day, the interferometer's primary virtue is its incredibly precise measurement of wavelength. Michelson next used his invention to calculate the velocity of light, arriving just before his death at a figure within 11 miles (17.7 km) per second of today's most accurate calculations.
The Michelson interferometer is commonly used in astronomy to determine the size and separation of distant objects such as stars. Michelson himself pioneered stellar interferometry in 1920, when he used a large interferometer mounted on a California mountainside to measure the diameter of the star Betelgeuse in Orion.
Interferometers can be found in several different incarnations, designed to perform a vast array of tasks. The Twyman-Green interferometer uses a point-source of light, typically a laser beam rather than white light, and is often used to measure surface smoothness. The most commonly used interferometer is a modified Twyman-Green model.
In 1916, the French physicist Charles Fabry (1867-1945) used an interferometer of his own construction to measure the quantity of ozone in the Earth's upper atmosphere. This interferometer differed from the others in that it used more than two beams. Fabry's findings provided a basis of research that helped establish meteorology as a legitimate science.
The Mach-Zehnder interferometer is an example of a very specialized version of the apparatus. By adding two cells through which the perpendicular beams are directed, this interferometer becomes ideal for observing airflow around models of missiles, aircraft, and other projectiles.
Metrology is an important application of interferometry. Optical component manufacturers use interferometers to precisely measure the shape and surface smoothness of lenses and mirrors. With the devices' unsurpassed precision, deviations of even one wavelength can be detected. Such precision is essential in the construction of sensitive optical equipment, such as telescopes and microscopes. Interferometry has applications in other fields such as semiconductor manufacturing, for example, where deviations of a fraction of a micron can significantly reduce device yield. Interferometers are used to inspect silicon wafers for smoothness and flaws prior to circuit fabrication.
One of the newest aids to the science of interferometry is the hologram. Designed to create a permanent three-dimensional recording of an interference pattern, a hologram is almost like a frozen interferometer. Scientists reconstruct the interference fringe stored in a hologram and cause it to react with a new comparison wave. The fact that the holographic pattern can be stored and reused makes this method more advantageous than common optical interferometry.
The interferometer was Michelson's most significant single contribution to the world of science. In 1907 he was awarded the Nobel Prize in physics for his dedication to the design of precise scientific equipment. He was the first American to receive the prize in one of the sciences.
This is the complete article, containing 784 words
(approx. 3 pages at 300 words per page).
