Science Buddies: "Ask an Expert"

Hello,

An important property of waves is interference. Can interference occur between a mechanical wave and an electromagnetic wave?

islamfaisal wrote:Can interference occur between a mechanical wave and an electromagnetic wave?

Not directly; however, if the mechanical wave is altering a surface that is affecting electromagnetic wave reflection or transmission, then there is a definite indirect interference.

There is at least one very common example where equipment is intentionaly designed to make use of this in most grocery stores. The laser bar code scanners in checkout lines typically have mirrors mounted on a rotating axis that is also being tilted so the beam will create continuously changing scan patterns that have a higher probability of being able to read a bar code at an arbitrary yaw and pitch at an arbitrary presentation angle.

Most radar used by air traffic control of aircraft consists of a rotating antennea which is another example that has been around for years.

-Craig

Craig gave you some examples of interaction between mechanical devices and E-M waves. If, however, you want actual interference such as seen in a two-slit interferometer, then I can't think of an example. I'm not even sure the concept of interference between a mechanical wave, e.g. sound, with an E-M wave by means of vector addition of the wave amplitudes to produce constructive and destructive interference makes sense, because you can't really "add" a mechanical displacement and a voltage. It is possible to get coupling between a mechanical wave and an EM wave, as found in Alfven waves in a plasma or in devices such as a molecular beamline or a klystron, but these are not actually interference in the optics sense of the word.

Craig_Bridge wrote: The laser bar code scanners in checkout lines typically have mirrors mounted on a rotating axis that is also being tilted
-Craig

Thanks for your interest. Just one question: How is the mechanical wave represented in this example? I didn't get it totally. Thanks again for your interest.

John Dreher wrote:I'm not even sure the concept of interference between a mechanical wave, e.g. sound, with an E-M wave by means of vector addition of the wave amplitudes to produce constructive and destructive interference makes sense, because you can't really "add" a mechanical displacement and a voltage.

Thanks for your reply. Can I generalize this with superposition also, right?

[quote="islamfaisal]How is the mechanical wave represented in this example?[/quote]
"mathmatically" One way to mathmatically think about any mechanical wave is to consider it describing the position or angle of something that cyclically changes with time. The fact that it repeats makes it a wave. The rate at which it repeats is a frequency. In the case of the rotating and tilting mirrors, there are two fundamental frequencies. The time it takes for the mirror to rotate around the axis it the basis for one frequency and the time it takes for the tilt angle to go through its entire range and return is the basis for the other. Typically there are 4, 6, or 8 identical mirrors on the rotating head, only one of which is reflecting the beam at any given point in time. This effectively is a frequency multiplier for the rotational frequency because the effect of all the mirrors is identical in terms of reflection angles wrt the laser. The mathmatical wave description of the reflected lazer beam rays is a very complex mathmatical equation.