Re: What cause's an afterimage?

Your question is in two parts 1) What is an afterimage and 2) How do the 
cones in the eye work.  I'll answer your second question first, then attempt 
the other!

There are two types of photoreceptors (neurons sensitive to light) in the 
retina of the eye: rods and cones.  They get their names from their shape - 
cones are conical and rods are, well, rod-like!  Cones are specialised for 
day vision and are sensitive to colour, whereas rods are specialised for 
night vision.    

Light (the visual image) that has been focused onto the retina by the 
cornea and lens is absorbed by visual pigments or chemicals located inside 
the photoreceptor.  The cascade of biochemical events that follow is termed 
'phototransduction'.  Here the light signal is transformed into a kind of 
neural 'Morse-code' that can be passed onto other areas in the brain for 
translation into the image that we see.  

There are three types of cone that transmit information about different 
wavelengths of visible light.  The visible spectrum of light (all the 
colours that we can 'see') ranges from about 400 to 650nm.  Cones are 
sensitive to either blue, green or red light and send out neural impulses 
when they absorb light of that wavelength.  All colours can be sensed by 
various combinations of these three colours.  Its important to note that 
the 3 primary colours of 'light' differ from the three primary colours red, 
blue and yellow you mix with paint. E.g. red and green light mix to make 
yellow and all three colours mix to make white.

Cones transmit their signal to ganglion cells that send the signal along 
the optic tract to the visual centres of the brain.  Ganglion cells receive 
input from 'opponent' cone cells and calculate the difference in signal 
between red versus green inputs and blue versus yellow inputs. The signal is 
sent onto the visual cortex of the brain where it is matched up with the form 
and movement of the object being seen.

Afterimage is the phenomenon you notice when you look at a red object, then 
at a blank white wall and you see the object as green (or a blue object as 
yellow, or a white object as black). Here the red cones tire (or habituate) 
from sending so many 'red' signals.  So when you look at the white wall the 
green cones are not tired and send some signal (remember that white light 
contains all colours) and a 'green' signal is transmitted to the brain 
instead.

References: 
Kandel, Schwartz and Jessell. 1995.  Essentials of neural science and 
behaviour.  Appleton & Lange.  

Ganong.  1995.  Review of Medical Physiology.  Appleton & Lange.