Quote:
Originally Posted by pj.rodriguez
monochromatic... Doesn't that mean that if you shine light of the same wavelength at this light, it will cancel and effectively have no light shining?
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Not exactly, it means all the light has the same wavelength so if two photons are 180 degrees out of phase with each other they will cancel out. Now lasers have another property which is that the light created all has the same phase so if you shine one at a reflective surface that was perfectly flat and the distance was such that it caused the reflected light to be at a 180 degree phase shift it would cancel.
In practice because light wavelengths are very short and surfaces are not perfectly flat you get Speckle (
http://en.wikipedia.org/wiki/Speckle_pattern) which isn't full cancellation but is quite horrible to look at.