What is the smallest difference in wavelength that we can percieve as a difference in colour?

Question

If yellow light can be made from a mixture of red and green lights, how does the brain interpret this as yellow? Can we percieve photons intermediate in energy between red and green, and if so, how? In other words, is there a difference between 'pure' yellow, and mixed red and green?

Answer 1

The human eye has three kinds of cells, each sensitive to a different frequency of light. For the record, one is at peak sensitivity at 440 nm wavelengths (bluish), one at 544 nm (green/cyan), and one at 580 nm (red). Each of those is sensitive at a range around those wavelengths, so there aren't any wavelengths between around 380 to 750 nm that don't stimulate one or more of those cells to some degree. Your brain takes those three groups of cell stimulation signals and decodes it into one continuous colour.

Because of the way your eyes send signals to your brain, there is no way for the difference between a stream of pure yellow photons and a mix of red and green photons to be sent. That information is lost at that stage. Allow me to demonstrate using completely made up numbers:

If you use a mix of red and green, it's pretty straightforward. The red photons will stimulate (more) red cells, and the green (more) green ones (in reality, red cells are about 80% as sensitive as green ones at green light, and vice versa - there's a lot of overlap!). Your collection of 5 bursts of red and 5 bursts of green stimulate 5 red cells and 5 green cells. All good.

Suppose your yellow was exactly between red and green. It has a chance of exciting both red cells and green cells, but neither of them very vigorously. Your 10 bursts of yellow stimulate BOTH the red and the green about half the time, so you get 5 red signals and 5 green signals. The same.

The cells can tell, but the information is lost before your brain gets a hold of it! Given the way these things work out, though, I'm not sure the brain would have been that careful with the information anyway.

Hope that helps!

Answer 2

my wild guess would be 1dB. That's what we can perceive as a difference in sound level. This would be 26%. So I would propose that the frequency needs to change by 26% in order to see a colour difference.

Here is a table of frequency vs colour from wikipedia
Red 4.28
Ora 4.84
Yel 5.17
Grn 5.66
Blu 6.38
Vio 7.14
where the freq are 10^14 Hz.
The differences between these colours varies from 9% to 13% so it looks like our eyes discern colours better than our ears perceive loudness changes. Oh well it was an idea.

These are just 6 colours. Of course we can all see more than that. So the question is just how many can we see?

I have a feeling that colour difference sensitivity will vary across the spectrum too.

Answer 3

Better than a nm. Look at a chart of colors for paint, if you think you can see more than 300-350 colors, which is less than the number of different paint colors, then you can see the difference of a nm in wavelength.

Answer 4

words and languages evolve over the years, so I anticipate 'colour' to be the older version. And 'colour' to be the in demand style. They the two get an identical message for the duration of, so i does not be too confuzzled (my be conscious!).