|MadSci Network: Physics|
Hi, A sound wave is a travelling pattern of increased and decreased pressure. Pressure can be increased indefinitely but cannot decrease below zero. This means that air is a non-linear medium and that intermodulation can definitely occur. It does take sound pressure levels in the order of air pressure to produce this effect discernably. It is probably noticeable from 150dB SPL onwards, a level lethal to human listeners. Light is an electromagnetic wave (ie. a pair of perpendicular waves in magnetic and electrical fields that create and maintain eachother), and hence does not need a medium. There is no limit to electrical and magnetical field-strengths (in a vacuum at least), so no non-linear effects occur. There do exist optically transmissive media that have non-linear behaviour and in which distortion and intermodulation between light-wave occur. Such media are used, for example, to double the frequency of near-infrared laser light to make it green. Similarly, they can be used to build 3d displays by allowing a pair of laser beams to cross inside such a medium where the intersection becomes a new source with frequencies equal to the sum and difference of the two source frequencies. In such a medium, your red and green spotlights (given sufficient intensity) would probably produce violet and far infrared. Some more on this at http://cord.org/cm/leot/course06_mod11/mod06_11.htm Regards, Bruno
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