A short introduction in human vision biology
         
Light rays reflected by an object -for example here, a pencil- enter the eye and pass through its lens.
The lens projects an inverted image of the pencil onto the retina at the back of the eye.
Muscles attached to the lens can change its shape so that normally the image can be
projected optimal with respect to overcoming the distance to the perceived object.
  
In the retina the projected light must also penetrate two layers of
neuron cells (neurons) before reaching the rod- and cone cells at
the back of the retina. The first layer or front layer consists out of
ganglion cells whose long axons (fibers that transmit electrical
impulses to other neurons) form the optic nerve leading into the
brain. The second layer or middle layer is formed by a type of
neurons that are called bipolar cells. In the rod cells and the cone
cells, which are photosensitive cells, signals are produced that start
on their way into the brain through the optic nerve and reach a
central relay station, the LGN - the Lateral Geniculate Nucleus.
The place where all the axons come together to form the optic nerve
is called for an obvious reason the blind spot. The area of the retina
where vision is optimal is called the yellow spot or fovea.
Signals about particular elements of the image (of the pencil in our
example) then are transported to selected areas of the Primary
Visual Cortex, or the V1, of the brain. From there, the signals are
sent to "higher" areas of the cortex that process more general
aspects of the image (of the pencil) such as its shape, colour, or
motion (or no motion as virtually in our case).
The various parts of the retina are so to say 'represented' in these
areas of the brain. The part that corresponds with the fovea centralis
-the area of the retina perpendicular behind the eye lens where
vision is best- is equal in size with the area that corresponds with the
other parts of the retina. This part is divided in two sub areas:
a sensor visual central area, where colour, size, form, motion and
clearness are perceived, and a psycho visual area that almost
surrounds it (except in the front) where identification and spatial
valuation of the perceived images takes place.
     
Retina  -microscopic enlargement  ( 200 x )-
      
                  multipolar ganglion cells           |     | the rod cells
bipolar ganglion cells           | the cone cells
                           multipolar ganglion cells
            (light)                   (dark) pigment cells
   
Light intensity is perceived with the rod cells, with the cone cells 
colours can be seen. The fovea consists of only cone cells, about
40 000 / mm2. Away from the fovea gradually the number of cones
are decreasing and the number of rod cells are increasing. These
parts of the retina are especially in use in the twilight when there is
little light and so colours seem to have vanished.
In the sense part of the photoreceptors -the rod cells and the cone
cells- or photo sensors, a large number of filmy discs are piled up 
perpendicular to the cell axis. In a cone cell these discs are about
5 nm thick, in a rod cell about 3.5 nm. On the surface of these discs
lays the material (a special pigment called rhodopsin) that is
converted by light (energy) by which the start of the impulse is
produced necessary to see. The rhodopsin on the discs in the cone
cells is of three types particularly sensitive to violet-blue, green and
yellow-orange light. Each of these rhodopsin forms absorbs a specific
part of the projected light by which the action potential is produced to
perceive colours.
    

         Relation of wavelengths to cone receptivity:
     
Looking at these 'figures', for that matter, they might give you also
some thought why it seems so pleasant to look into a garden or any
landscape like that.
Of course many things in such a complex process can go different
from what could be considered as standard or optimally, due to the
laws of variation in life or evolutionary development. That some
of us actually do see colours differently has indeed a relation with
the photosensitivity of the light sensitive pigments in their cone cells
or even the missing of these pigments. (Just click here for some
more information about this subject).