As we learned before, different wavelengths of light are associated with different colors. However we should not equate the two, because while we associate a certain wavelength of light with a particular color, we may still see that color when the wavelength is not present. This implies that we can see colors which are not there, even those that are not in the visible spectrum.

Different people see colors differently. Most people will see blue color when wavelength 450nm to 490nm is presented to them. However none of them will agree exactly on the specific wavelength.

Most colors we encounter are not monochromatic (of one wavelength). For example, if we shine a flashlight on a blue piece of paper and decompose the resulting light via a prism, we will see that relected light consists of many wavelengths.

What you see in this picture is an intensity distribution curve. It shows the relative amount of light at each wavelength. When your eye sees two different wavelengths in the same place, it does not separate them into several colors. You actually see a mixture of the two. For example, if you see monochromatic light with wavelength 575nm you would see a saturated yellow (meaning a pure yellow color). However if you look at an object and it looks yellow, it does not imply you are seeing a wavelength of around 575nm. You could be really seeing one wavelength of 545nm and another of 680nm, which both look exactly the same to your eye. This means your eye is very easily tricked by light.

You may be wondering why your eye is confused so easily as in the example above.

The human eye uses 3 different photoreceptors to distiguish colors. Each photoreceptor is sensitive to a certain range of light wavelengths.

The diagram illustrates the sensitivity range of each photoreceptor.

For example: If your eye sees monochromatic light with wavelength 555nm, the green photoreceptor will be stimulated most and the brain will get the signal that the eye is seeing a green light.

The photoreceptors are wired together in an interesting way through different channels which send the final color sensation to the brain. Below is an interactive hypothethetical schematic illustrating the connections.