HumanisTim

This is something I've been wondering. What is the explanation for why, our eyes are able to adjust to a pitch black dark room after several minutes. My two guesses are either. A. Our brain begins remembering what the room used to look like and restoring images of it or b. no room is really totally dark and always has a little light and the brain uses that light to form the images as best it can. If any one can answer this thanks :-)

major_kong

Check this out.
(<a href="http://www.howstuffworks.com/question53.htm" target="_blank">http://www.howstuffworks.com/question53.htm</a>

tronvillain

In other words, it is your eyes that do the adjusting, not your brain. Of course, the brain may play a role as well.

Coragyps

One of the links from that link might go into this, but "totally dark" is a pretty rare thing. The average house leaks enough light under doors, etc., that it's pretty difficult to get even a closet really dark without a lot of towels and duct tape. One of the high points of the official guided tour of Carlsbad Caverns is when they turn out the lights back in some isolated passage. After about five or ten minutes in that sort of dark, an illuminated wristwatch nearly hurts your eyes.

liquid

Our eyes actually only require a few thousand photons to register some kind of light - they are very sensitive. As has already been mentioned, it is very difficult to get a room even dark enough for human eyes - not withstanding the propensity of light to get through any gap, photons can even transmit through 'opaque' surfaces due to quantum effects (although that's hardly on a significant level for visual perception!)

If you are interested in visual/neural phenomena, why not have a look at <a href="http://nivea.psycho.univ-paris5.fr/ASSChtml/ASSC.html" target="_blank">change blindness</a>

Late_Cretaceous

This is a topic familiar to any amature astronomer. Although it only takes a second for the pupil to make the adjustment from light to dark, the retina takes up to 45 minutes for "full dark adaption". The retina produces a pigment (rhodamine) that allows the rods to respond to low light levels - or something to that extent. Rods are more common in the periphery of the retina then in the center (which is dominated by cones). Amature astronomers will use red LED flashlights in order to read star charts at night because rhodamine is not affected by red light- but look into a pair of head lights at a distance and your dark-adaption of ruined for several minutes. Rods are not color sensitive, like the cones are. This also explains why every thing in a telescope looks muted greenish-gray instead of the vibrant colors you see in photos. Another phenomenon with the dark-adapted human eye is caused by the relative paucity of rods in the center of the eye which results in the fact that faint stars appear to jump out when you look away and fade when you look directly at them.

One more thing. Even on a totally moonless night, there is enough light from the stars to be able to see at night.

[ June 30, 2002: Message edited by: Late_Cretaceous ]

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Writer@Large

They did the same demonstration in Mammoth Cave National Park. It was very cool to experience such total darkness.

-W@L

Undercurrent

I had heard our eyes can tell the difference between total darkness and only a few dozen photons.

m.

One of the last sane

I am told that they do the same thing on at least some tours of the Catacombs under Rome.

Edit: typo

[ June 30, 2002: Message edited by: One of last of the sane ]</p>

Christopher Lord

Thats not the whole story either.

A sheet of paper in moonlight casts far fewer photons on the eye than a hunk of pitch-black coal at high noon by an order of magnitude.

Yet the paper is clearly 'white', and the coal 'black'.

This indicates a powerful whitebalance system in the brain, far better than the best algorithims used in cameras.

Not only that, we have some fancy color-balance features as well. Our color vision is only tinted by extreme situations, like sodium lamps. most of the time you can call colors as they are, even under extreme situations like colorful sunsets, which makes red, yellow, and purple very difficult to identify.

This hints at how powerfully subjective our vision system is.