INTERFERENCE OF LIGHT: Applications to Interferometry CourseGrade 12 Physics Time RequiredTwo 1-hour lessons PA Standards Met C: Evaluate wave properties of frequency, wavelength and speed as applied to sound and light through different media.

Principles Since light has wave properties, it will experience interference (the addition of waves). This interference is like that seen with water waves. Whether you get constructive or destructive interference depends on the wavelength of the light. White light is made up of many colors. These different colors have different wavelengths.

Interference as commonly observed: We can see the many colors of light on soap bubbles. The separation of white light into many colors on a bubble happens because of interference.

Chromatic interference is seen in nature, such as in sea foam, which is made of plankton. The following is an animation of interference of waves coming from two point sources:

Green metallic-like reflection of a photographic flash from the eyes of a cat, caused by multiple thin-film interference.

Scales of stomatopod crustaceans viewed with different angles of a polarizing filter. Photos taken a few moments apart

Making Use of Interference of Light: Interferometry is a technique of using the pattern of interference created by the superposition of two or more waves to diagnose the properties of the aforementioned waves. Interferometry makes use of the principle of superposition to combine separate waves together in a way that will cause the result of their combination to have some meaningful property that is diagnostic of the original state of the waves.

This works because when two waves with the same frequency combine the resulting pattern is determined by the phase difference between the two waves: waves that are in phase will undergo constructive interference while waves that are out of phase will undergo destructive interference. A B A = Combined wave form of initial waves in phase B = Destructive interference resulting from waves out of phase.

How an Interferometer works: The instrument used to ‘interfere’ the waves together is called an interferometer. Most interferometers use light or some other form of electromagnetic wave.

Illustration: Mach-Zehnder Interferometer

Interferometry is an important investigative technique in astronomy, plasma physics, fiber optics, engineering metrology, optical metrology, oceanography, seismology and quantum physics. The following is an interference pattern produced with a Michelson Interferometer:

Interferometer: Application to Nanotechnology When the wavelength of the light used is known, small distances in the optical path can be measured by analyzing the interference patterns produced. This technique is used to measure the surface contours of materials, making it possible to study materials at the nanoscale level. Nanotechnology has been used to produce novel structures and films with wide and useful applications.

Who makes Interferometers? University of Pennsylvania, SEAS Lab: Uses one supplied by the Zygo Corporation. They conduct lots of nanotechnology experiments.

LAB ACTIVITY: ‘DESIGNING’ AN INTERFEROMETER SCHEMA 1:

SET-UP:

ACTIVITY: “HOMEMADE INTERFEROMETER FOR $ 20” Adopted from “Metacafe Video Entertainment / Celtic Mad Scientist” AV DISPLAY

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