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Log, Apr 25, 2008

12 people. Well we really tried but the weather just would not cooperate with us. It was clear all day but just enough of a cloud layer built to ruin any chance of seeing anything. Still we had quite a nice time talking about science both with the half dozen kids who showed up and the adults. One topic we about which Les went into long detail was the role of the human eye in observations. All this came about when one of the adults asked why pictures of deep space objects are often in color while the view we get is generally gray with an occasional bright star showing traces of color.

The answer is a little more complex than you might think. There are several reasons. First of all most cameras record their view over an extended period of time while the eye is essentially instantaneous. This means the cameras can accumulate enough light to show colors while the eye only has enough time to get a fleeting gray tone Image.

There is a second part to the answer - the very way that our eye works. Lots of people know that the human eye is primarily responsive to three basic colors: red, green and blue. The brain (not the eye) makes a judgment about the relative intensities of each color and it produces all the other tints, shades and primary colors we see. It is also the reason why monitors, TVs, and the host of other electronic devices that show us colored images are usually RGB [yup - red, green and blue]. I wondered out loud about what it must be to be poor old Fido looking at a TV. Dogs see primarily in a single color - yellow which really does not match a TV very well.

However beyond the three daytime colors, there is a fourth color which we see - but only in dimly lighted places after our eye adapts to the dim light. This is actually a shade of light between green and blue called cyan. Yet at night, we don't see a greenish-blue color but a shade of gray. How come? Well, again the eye only picks up photons. It is the brain that actually converts what they eye reports into a color. For humans, dim light is always reported as a shade of gray. The light sensitive material [dye] for night viewing is a compound called rhodopsin. [This literally translates from Greek as purple stuff.] This dye is different than the dyes for red, green or blue. First of all it is far more sensitive to light than the colored dyes and secondly it bleaches clear when light levels get high. Ever left a dark room and entered the daylight only to be dazzled. That is the rhodopsin being bleached clear and turning off. It takes about 15-30 minutes of darkness or red only light for the rhodopsin to turn back to purple and become turned on.

The rhodopsin is mainly in eye cells called rods while the red, green and blue dyes are mainly in another kind of eye cell called cones. The cones are centered in a small dot very densely at the center of the eye called the fovea. The rods are in a ring around the fovea. This is why the dark adapted eye sees dim objects best when you look to the side of the object. This is called averted vision and is one of the skills serious astronomy buffs practice to see more.

-Les Coleman

Leslie Coleman
Author:
Leslie Coleman
Entry Date:
Apr 25, 2008
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Leslie Coleman's Log
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