# Question about light waves and phases.

## Main Question or Discussion Point

I am a sound tech and I no from my work there that if you have (in theory) two sound waves and 1 is 180 degrees out of phase with the other that they will cancel each other out. - This is mainly the basic theory of noise canceling headphones (along with some acoustic material, etc) and balanced mic leads.

But if we model light as waves does that then mean that if there was two identical sources of light (same wave length, amplitude, etc) but one was 180 degrees out of phase with the other would that appear to produce no light at all?

Just a thought, that no one seems to be able to tell me the answer to. Maybe I'm missing some information somewhere along the track.
Any thoughts?

Thanks
--
Andrew Bob

## Answers and Replies

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cristo
Staff Emeritus
You are correct. The phenomenon is known as destructive interference, and works for light as well as sound.

Would it be possible to build such a light?
So that you could have them on side stage constantly changing to the lighting conditions on stage and just keep canceling out the light? Like noise canceling headphones?
Or is this where the real world starts to make theory's stop working?
--
Andrew Bob

ZapperZ
Staff Emeritus
The difference here is that sound has a significantly longer wavelength than light. So you can make sound to have a region of "destructive interference" larger than sound. It is not possible with light because, if you have seen the pattern from a 2-slit experiment, you'll notice that in the visible range, the region where such destructive interference occur is VERY small.

Zz.

But if we model light as waves does that then mean that if there was two identical sources of light (same wave length, amplitude, etc) but one was 180 degrees out of phase with the other would that appear to produce no light at all?
Yup. There are a number of phenomena that demonstrate it.

IIRC, The "anti-reflection" coating on monitors and camera lenses is a transparent film that is a half a wavelength in thickness. Light from the outside (the glare) bounces off both the initial (outer) surface of the film, as well as off the inner surface (or maybe it's the outer surface of the glass that it's coating ... whatever). You can then treat the different reflections as if they were "identical sources of light (same wave length, amplitude, etc)". Since the second reflection is 180 degrees out of phase with the first reflection, it cancels out. Of course, that "half wavelength" is only true for the middle of the visible spectrum, so the upper and lower visible frequencies are still visible in the reflection, hence the blue and red appearance of the anti-reflection coating.

Oil slicks on water also produce their rainbow effect in a similar way. The different colours come about due to slight variations in the film thickness, so that different wavelengths of light are cancelled out in different atreas of the oil slick

Just for laffs, see also http://en.wikipedia.org/wiki/Double-slit_experiment .

rdx
Would it be possible to build such a light?
So that you could have them on side stage constantly changing to the lighting conditions on stage and just keep canceling out the light? Like noise canceling headphones?
Or is this where the real world starts to make theory's stop working?
--
Andrew Bob
One way to demonstrate destructive interference at optical freqs is to use a laser to get a monochronic source. Then send your monochronic beam to a double slit which will act like two sources of the same light in phase. The result will be an interference pattern where the light will alternately interfere constuctively (bright bar) and destructively (dark bar). The analog in sound would be to have a sound of a single note coming from two speakers properly spaced and in phase. Walk across in front and hear the intensity rise and fall alternately.

Claude Bile