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# Physics/How does diffracted red, green, blue light look like?

Question
QUESTION: How does diffracted red, green, blue light look like?
I know that once refracted, white light emits the colors red, orange, yellow, green,blue, indigo and violet but i cannot seem to determine the colors it emits once the light is going through a plastic filter (in this case the 3 primary colors) could you please tell me the pattern these three colors produce (ex: ROYGBIV for white)? Thank you.

ANSWER: Actually, this is all wrong.  White light splits into a continuous range of colors.  In other cultures they don't define their colors the same way they're split up in English.  In Chinese they have something more like 9 major color words, for example.  It's a continuous spectrum and not something you can categorize like that scientifically.

The filters question is even more wrong.  Filters don't split light, they absorb all the colors of light that are not the one that's passing through, usually.  I say usually because a green filter could pass light in the spectrum region that you would perceive as yellow and as blue.  That would excite both the short and medium cones in your eye, causing your brain to perceive it as green even if it absorbed green light.  The issue is way more complex than the question even gets to.  To know more, study the perception of color and the way your three types of cones can see it.  Understand, there are animals out there with far more complex eyes than we will ever have, like the mantis shrimp (17 different types of cones, if memory serves).  Then you'll understand what you see in a "different light."  Pardon the pun.  :)

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QUESTION: i understand but then you mention that the spectrum for green could also contain some yellow and blue; if so, then i assume the spectrum for blue could contain green and purple but i cannot determine the red one, could it contain orange and yellow?

ANSWER: No, I'm saying that what you *eye* could see as green might contain no green at all.  There's a difference between what you see and actual scientific spectrum.  Blue is pretty much just blue, it only excites the short cones in your eye.  Read up on wavelength and you'll understand better.  Red cannot contain orange or yellow because it's at the end of the visible spectrum and you can only excite the long cones in your eye with red.  Higher frequencies such as orange and yellow would excite the shorter cones and be perceived as different colors.

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QUESTION: i see, here is the thing, i tried it out by using a flashlight, diffraction goggles and color filters (red, blue and green). my results for blue spectrum showed the colors blue, green, a small dot of red (and maybe purple, not really sure). for green and red it gave me all the colors (the 7 in the rainbow except for indigo) and i am not sure this is correct. What i would like to know is what colors should show up in one of the spectrums for each of the filters?

Diffraction goggles, like a diffraction grating built into actual goggles?  How novel!  Anyhow, your eyes respond logarithmically to light intensity, so it's no surprise that a filter that would look blue to your eye straight-on would have hints of a few other colors sneaking through.  The green is also not a surprise, as your eye had to pick a mid-wavelength frequency to interpret.  The red is a bit more mysterious, but surely the red wavelengths would seem to be more intense than the other ones.  I suspect that you had filters with dominant colors and less-intense colors, but until you split them with a diffraction grating your eye would be swamped by the primary color.  Without seeing the filters, I would have no way to test it.  Unless you have some specific test equipment, an extremely detailed photo of everything you described, or a place I can find these filters, then I can't say for sure...there aren't enough details for me.

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#### Dr. Stephen O. Nelson

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