This expression which we find traces from the fifteenth century, warns that without good people "light" on all the facts situation can quickly become confused ... but it is also a scientific observation that can easily explain. Indeed in low light at night, we no longer perceive the colors ... And yet they still exist, the white light of the moon for example, containing all wavelengths. Explanations ...
The bottom of our retina is a surface light-sensitive photoreceptor cells lined because of two types: rods and cones. These cells are composed of numerous membrane folds or disks. It is embedded in these membrane discs that are the "photosensitive pigment protein," the secret of our vision ... [Indeed, when a photon hits one of these proteins change conformation, allowing the transmission of a signal (via a G protein), it follows a change in ionic currents which leads to hyperpolarization: departure of an electrical signal which will be processed by the brain ...]
But I digress, to understand a little detail these two types of cells ...
cones , numbering 5 million per retina in humans are mainly located in the center of the retina. There are three types according to the pigment present in the disks, each specializing in a wavelength : Blue, green and red ... [* 1] They are responsible for color vision and their density because they are also responsible for visual acuity (day anyway) ... The number disc membrane is still relatively low (approximately 100), there is so little photosensitive pigments in these cells.
sticks , are much more numerous (125 million men) and located somewhat on the periphery. There is only one type, whose pigment, rhodopsin has an absorption maximum at 496nm (blue-green). The difference is that we are interested in the number of membrane disks whose number is approximately 900 per cell. The photosensitive pigment is much more present and the light sensitivity of this cell type higher (1000 times higher than for a cone).
consequence of this, a low light, only the most sensitive cells into action: the sticks. The result is a vision in black and white with good peripheral acuity (and Yes the night vision is better in the periphery ...) is the scotopic vision (Greek Skotos, dark), as opposed to photopic vision. And that is why all low-light cats appear to us in monochrome ...
You need a guinea pig (human), some crayons and a sheet of paper ... Sit your guinea pig, and ask him to look straight ahead, locate then the limits of his field of vision by moving your finger from the center outward until the pig can not see your finger.
Then take a sheet of paper or a screen behind which you hide the pencil and hold it in the periphery of vision. Then show a pencil at random and ask what color your guinea pig it is (it has a high chance of being wrong).
Repeat with another color to increase the statistical power of the test and its humorous effect ...
Some figures on light sensitivity:
- The human eye reaches a sensitivity of 10-14 W, which corresponds to the impact of a few photons, or light intensity of a candle at 16 km ...
- After 45 minutes of darkness is at its maximum sensitivity (about one million times higher than immediately after leaving an illuminated area) because all rhodopsins have regained their starting conformation. Between
- absolute black and white, the human eye can distinguish about 30 to 40 different gray levels.
- Color blindness is a visual defect color associated with abnormal cones (mostly red) affecting 8% of men and slightly less than 1% of women.
For more information: photoreceptors on the site "the brain at all levels"
[* 1]: Current research indicates the existence of a fourth type of cones sensitive to orange in 10% of men and 50% of women.
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