Usually, when asked whether the purple color exists rainbows, an answer similar to this is given: The purple color is perceived by human eyes via the activation of both red-sensitive and blue-sensitive cone cells. It is known that purple isnt a physically monochromatic light (a light composed of a single wavelength). However, on diffraction gratings, the spectrum repeats itself in such a way that part of a lower-order diffraction overlaps with that of a higher order. For example, it is conceivable that the red band of the second order diffraction overlaps with the blue-violet band of the first order diffraction, producing the purple color perception. This would also occur in thin-film light interference, such as anti-reflective coatings of eye glasses, etc. Could the same occur in rainbows, which is dispersion in water driplets, where red and blue-violet somehow overlap? The overlap that you might see in a diffraction grating doesnt occur in a rainbow, because rainbow are formed by refraction of light, not diffraction. The raindrops work like prisms, not diffraction gratings: its a simple bending of the light by a frequency-dependent angle. There are no multiple orders of maxima to overlap. For what its worth, though, a lot of people dont make the distinction between purple and violet (which does occur in a rainbow).
Posted on: Fri, 28 Mar 2014 18:48:46 +0000
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