Physics Question #2158

Darren Connell, a 33 year old male from the Internet asks on June 28, 2004,

We have the ability to use sound frequency to cancel out unwanted sounds, and since light is a frequency is there a way to use the opposite of visible light frequency to cancel out visible light? And if so would this make something invisible?

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The answer

James Hope, A-level student, England answered on December 26, 2004

Yes it is possible, but your idea is slightly wrong, it is not the opposite frequency, but exactly the same frequency but 180° (or half a wavelength) out of phase, so that a peak of one light wave would coincide with a trough of another light wave. This same effect can be seen on soap bubbles, where certain colours are being 'cancelled' leaving others, by the reflection off the top surface being out of phase with the reflection of the bottom surface. The two problems are that light reflected off an object is rarely one colour, and so is more than one frequency of light, and would therefor need very complicated apparatus to make sure every colour is in fact completely cancelled. also it would not make something invisible any more than something with no light reflecting off it is invisible. It would just appear black.

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