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hyperbyte
2009-Mar-24, 10:04 PM
i was talking about photons today and got really confused with this scenario.

you are in a completely dark room. you have a blue torch. there is a yellow chair. if you shine the torch on the yellow chair what colour would it be?

i thought it would be green but if you are shining blue photons and the chair only reflects yellow photons then where does the green come from?

01101001
2009-Mar-24, 10:28 PM
i thought it would be green but if you are shining blue photons and the chair only reflects yellow photons then where does the green come from?

Rare would be the fabric that only reflect yellow frequency. Likely it would reflect some in the blue too and that's what you'd see. But as you describe it, completely absorbing blue, it won't be green but dark, tending to black.

Residents of areas that use low-pressure sodium streetlights that cast a narrow band yellow-orange light, are well aware of the effect it has on objects that don't reflect strongly in yellow. Pity the poor people of Silicon Valley, California, who make the lighting sacrifice for nearby Lick Observatory, having to be extra careful when parking at night to see if they are parking next to a red-painted curb. Printed no-parking signs had to be added to reduce confusion.

cjameshuff
2009-Mar-24, 10:32 PM
you are in a completely dark room. you have a blue torch. there is a yellow chair. if you shine the torch on the yellow chair what colour would it be?

It would likely appear black, but if there is fluorescence involved it may be another color.

Individual photons have a single wavelength, and whether they are reflected or absorbed depends on that wavelength. Colored objects effectively just "sort" the wavelengths of the incident light. A highly saturated yellow object will reflect photons in the red to green ranges, and absorb those in blue.

It's even possible that it reflects in a narrow band between red and green, and would appear black in pure red, green, or blue light, or that it reflects sharp peaks in red and green and, though appearing the same color to the eye in white light, would appear black to 580nm "yellow" light. This is why sodium lights give poor color rendition...they appear pinkish-yellowish white, but actually have a spectrum with sharp peaks which "miss" the reflection peaks of many pigments, making cars of drastically different colors look the same, often making them look black.

A fluorescent yellow chair may re-emit some of the energy from the absorbed blue photons as red and green. This is why UV "blacklights", which emit mostly light that the human eye is poorly sensitive to and can't focus, make fluorescent objects glow brightly. Many things will fluoresce in blue light as well, though it's generally harder to notice.

Jeff Root
2009-Mar-24, 10:46 PM
A material which is yellow usually reflects green and red light as well
as yellow. The combination of green and red light look yellow to the
human eye.

Shining a pure blue light on a material which is pure yellow (or any
combination of red, orange, yellow and green, but no blue) will result
in no visible light reflected from the material. All the blue light will
be absorbed. If you shine a blue light on a yellow chair, and the
chair does not look dark, chances are the yellow color has a lot of
white in it, meaning that it does reflect blue light. Compare that
yellow to a deeper, more intense yellow, and you can see that the
lighter yellow looks whiter. That is because it reflects some blue
along with the red, yellow, orange, and green.

Human color vision is very quirky, because nature didn't design it
with any plan. It developed very haphazardly. So we have three
color receptors. One is most sensitive in the part of the spectrum
that we consider to be red, another is most sensitive in the green,
and the third is most sensitive in the blue. Combining a large amount
of red and a little yellow or green light gives orange. Combining equal
amounts of red and green light gives yellow. Combining equal amounts
of green and blue light gives cyan, or blue-green. Combining equal
amounts of red and blue light gives magenta, or purple.

Combining different colors of light is adding those colors together.
Reflecting light from a colored material is subtracting color from the
light. For example, when you shine white light (all colors) on a yellow
chair, the yellow material absorbs the blue light, but reflects the red,
orange, yellow, and green light.

Here's a web page I made that might shed some light on color mixing:

Color Wheels and Triads (http://www.freemars.org/jeff/colors/)

-- Jeff, in Minneapolis

nauthiz
2009-Mar-24, 10:50 PM
For added pointless fun: If you're talking about the pure colors blue and yellow, they don't mix to make green.

In additive (RGB) colors they make white, because in that system yellow is actually a secondary color you get from mixing the primary colors red and green. Adding blue to it gives you all three primary colors mixed together, which is white.

In subtractive (CMY) colors they make black. It's a similar situation, except now it's that blue is the secondary color you get from mixing cyan with magenta. Adding yellow makes it all three primary colors together again, which produces black.


The reason things don't normally work this way in real world situations such as when you're mixing paints is that most pigments are "dirty" - they reflect light in a lot of different wavelengths. This means the result you get from mixing two pigments is the result of a more complex interplay than what's represented by the two idealized color models. (Also, "blue" paint is frequently more cyan than blue - cyan mixed with yellow does make green.)

cjameshuff
2009-Mar-25, 12:01 AM
Human color vision is very quirky, because nature didn't design it
with any plan. It developed very haphazardly. So we have three
color receptors. One is most sensitive in the part of the spectrum
that we consider to be red, another is most sensitive in the green,
and the third is most sensitive in the blue.

It's even quirkier than that. There's also rods that are not themselves specific to a color, but can detect overall brightness independently of the cones and so have a minor role in color vision. And the distribution of rods and cones varies, there's more rods and very few cones away from the center. And there's blood vessels in front of it and a blind spot in each eye, plus the receptors get "tired" after being stimulated for a time, so the brain constructs an internal image of the surroundings as the eyes move around...and compensates for color casts in the ambient lighting. Take a "warm" light bulb and a "cool" one...with just one lit, you soon take the output as "white". With both lit...neither appears white.

What you see is a highly processed and extrapolated version of what's really there.

Noclevername
2009-Mar-25, 02:40 AM
i thought it would be green but if you are shining blue photons and the chair only reflects yellow photons then where does the green come from?

There are no "blue photons" or "yellow photons". What makes light appear different colors is the frequencies of the wave pattern; even blue light from a common light bulb passing through blue plastic has some green and yellow in it. The details will depend on the brightness, quality and type of the lightbulb, as well as the quality and type of the blue filter, and the specifics of the reflectivity of the chair's fabric. And, of course, how good your eyes are. ;)