Why does our eyes see red when exposed to too much light? When I looked into my projector when it was on the blue screen it left a red spot in my vision. I should not have tried it but all the colors left a red spot. Why not a blue or yellow spot was left?
 A: There are two effects at work here to form the Color Afterimage.


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*The blue light stimulates the S-type cone cells most, and they simply "tire out". Local supplies within the cell of ATP become run down, and the cell cannot therefore signal as often or as effectively. When you looked away into a more "balanced" light, white light that would normally fire all three S, M and L cones roughly equally now fires mainly M and L, because the Ss are temporarily run down.

*The phenomenon of neural adaption is happenning. In the presence of a skewed and lingering stimulus, the brain "recalibrates" its notion of "normal", so that the lack of skew if the stimulus is removed is perceived as a shift away from normal. This effect can be demonstrated in many ways: if you walk into a room with a strong smell, you'll notice it seems to disappear after a while, or at least become not so mingy. There's the fun experiment to show to children where you stand in a doorway with arms straight down, lift your arms so that they stay straight but stay in the transverse plane until your hands contact the doorframe. Now thrust in this position with your hands against the frame for about a minute, then relax and walk away from the doorway. Your arms will naturally lift even though you don't feel as though you're moving them: your neurally readapted proprioception has calibrated itself to believe that a force/torque needs to be imparted to the arms to keep them steady, when this is no longer true.
All of these effects, including the aftereffects of staring at bright light (free of UV and below $1{\rm kW m^{-2}}$ intensity), are fleeting and will right themselves naturally. There is no risk of lasting damage.
