Why do you need at-least three colors to make every other color?

Avichal

Since with the help of two different wavelength of light we can make every other wavelength of light why do we need three then - like red, green, blue or red, green, yellow? I guess red, blue or red, yellow will suffice.

tiny-tim

Hi Avichal!
nope

you can't make a single wavelength out of any other wavelength(s)
humans need three, because that's the way the human eye works

(i think some humans actually need four … so they sometimes see two things as different colours although most humans see them as the same colour)

each colour receptor responds to all wavelengths, but in unequal amounts

the response is a sort of bell curve, with a maximum in the red green or blue

see http://en.wikipedia.org/wiki/Color_vision for details

D H

To elaborate, suppose you have a white wall, a red laser, a yellow laser, and an orange laser. Aim the red and yellow lasers at the same spot on the wall and aim the orange laser at a different spot on the wall. Next tune the intensities of the red and yellow lasers so that the colors of those two spots is indistinguishable to the human eye.

Just because those two spots are indistinguishable to the human eye does not mean that they have the same wavelength. That they appear to be the same color is just an optical illusion based on how the human eye works.

dlgoff

And to add a little color to the thread



The Color-Sensitive Cones

NemoReally

... and not forgetting that many colours don't have a "wavelength" at all as they don't exist on the spectrum.

D H

E.g., purple. Or white.

1MileCrash

Thought it was pink..

But yeah, nothing to do with actual colors or light, just part of our biology. Our vision is trichromatic.

Avichal

Oh ok....its not the light or color. Its how our eyes work! So there is a possibility that some animals need only two colors i.e. they are dichromatic(or whatever it is called)?

snorkack

Yes.

Almost all mammals other than monkeys are dichromatic, like colourblind. Not monochromatic, though.

Avichal

Why is dichromatic colourblind?

NemoReally

In a normally trichromatic (or better) species, it means that the animal lacks the ability to see the full normal colour "spectrum". A dichromatic human is regarded as colour blind (even though they can see some colours) but I'm not sure that a normally dichromatic species can be properly be regarded as colourblind. Using that definition, then you could probably make a case for humans being normally colourblind as they lack the colour range of a tetra- or penta-chromat. Indeed, we have no idea what colour really is and, consequently, what possible values it could take. I think a monochromat could be regarded as colourblind as there is no indication that they see "colour" at all.

Alfi

Why do we need four colours to make a map, if we can only see three?

NemoReally

The 2 questions are not related. Stealing shamelessly from Wiki ..
The 4-colour map theorem states that no more than four colors are required to color the regions of the map so that no two adjacent regions have the same color, where two regions are called adjacent if they share a common boundary that is not a corner, and corners are the points shared by three or more regions. In fact, the word "colour" in this context is effectively just a label to indicate some unique marking (eg, letter, number or shading); for example, you could replace 'blue', 'green', 'red', 'yellow' with 'a', 'b', 'c', 'd', in which case you could call it the "4-letter problem"!

As for our vision, we can see significantly more than 3 colours - just open up the colour picking palette in Paint. As indicated in earlier threads, all things being equal, our eyes possess 3 specialized sensors (the cones) that have differing sensitivities to light of different wavelengths (energy, frequency, take your pick). Our brain then (somehow) combines the outputs from adjacent cones to construct "colour". A (speculative) reason we see so many colours is to aid identification of fruits, plants and other objects that appear visually similar in grayscale but are quite different in their edibility or danger.

D H

Pink is a non-spectral color, but a rather boring one. The same goes with light green, light blue, light anything. Pink is just light red, basically a white spectrum but with a bit of a peak in the red. (BTW, white is also non-spectral.)

Purple and magenta are much more interesting non-spectral colors. Suppose you shine a green laser and red laser at the same spot on a white wall. You'll see a yellow spot, a spectral color whose frequency is intermediate between the frequencies of the green and red lasers. A similar thing happens with a blue laser and a green laser. Now you'll see cyan, another spectral color.

Use a blue laser and a red laser and you'll get something very different. You don't see green, which is intermediate between blue and red in terms of frequency. You see magenta. Or purple. Or some other non-spectral color that in our heads is very, very different from green. Purple and violet are visually similar colors, yet spectrally they are very different.


And what it does to make the linear range from red to violet wrap around to instead form the color circle is part of that "somehow". Purple is a pigment of our imagination.

NemoReally

Ouch! If you send me your address, I'll arrange delivery of a smart bomb ...

Avichal

So can you say that a trichromatic species sees more colors than dichromatic and similarly higher i.e. tetra or penta see even more.

Now I feel as if I don't know what exactly color is?

NemoReally

In principle, simply on the basis that there is an extra 'dimension' for colour to extend into (by analogy, consider the rooms available in a single story dwelling to those in a high-rise tower of the same footprint and rooms per floor). However, the colour discrimination might be lower in a particular trichromat than in a dichromat (back to the analogy, d = 30 by 30 rooms and t = 5 x 5 x 5 rooms) as a result of cone sensitivity and / or brain processing. In addition, the colour dimensions might not contribute equally (eg, a 5 x 20 x 10 block or even (5 x 20) and (10) or some rooms might cover more than one floor).

... whether any of these happens in reality, I don't know.

I don't think anybody knows what colour actually is. However, we can develop mathematical models that allow us to functionally describe and model it. You might the following webpage of interest ... http://www.archimedes-lab.org/color_...llusions.html#