Naked Science Forum
General Science => General Science => Topic started by: eric l on 05/03/2007 16:40:09
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I do not know if this is a question of physics or a question of physiology, so I choose "general science"?
The question struck me (again) on occasion of the lunar eclipse a couple of nights ago. We had a guided walk in the park and adjacent wasteland, with a biologist and an astronomer as guides.
I remarked that, while all things seem reddish at sunset, they appear blue as the sun is completely down. I hoped the astronomer would be able to give me an answer, but that was not the case.
Anyway, it must be a well known phenomenon. Filmmakers used a procedure called "American night" to shoot night scenes in daylight using blue filters on their lenses.
So here is the question : do we see things in darkness as blue because blue light has more energy than e.g. red light (which would be physics) or because the cones that are receptors for blue are simply more sensitive than those that are receptors for red or those that are receptors for green (which would be physiology) ? Or is it still something else ?
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The fact things appear as reddish at sunset it's not surprising, since the color of the sunlight is reddish at sunset!
The fact things appears as bluish with little light is the "Purkinje effect":
http://en.wikipedia.org/wiki/Purkinje_effect
Is due to the fact that, with little light, retinal rods, instead of cones, are active, and they have their max sensitivity for the blue-green, instead of the yellow in the case of cones.
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The fact things appear as reddish at sunset it's not surprising, since the color of the sunlight is reddish at sunset!
The fact things appears as bluish with little light is the "Purkinje effect":
http://en.wikipedia.org/wiki/Purkinje_effect
Is due to the fact that, with little light, retinal rods, instead of cones, are active, and they have their max sensitivity for the blue-green, instead of the yellow in the case of cones.
There is a bit of contradiction between the cited Wikipedia article and the one on colour vision, which speaks of three distinct types of cones.
http://en.wikipedia.org/wiki/Colour_vision
Anyway, even there the higher frequencies are better covered than the low frequencies.
An other remarkable thing - but this would be psychology rather than physiology or physics - is that we "experience" red light as warm, and blue light as cold, while blue light is generally emitted by hotter sources - see also the "colour temperature" that photographers use to describe floodlights.
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I think some of both effects is due to the fact that blue light is scattered by the sky better than other colours so just after dusk everything is blue, because it most of the light you see has been scattered a few times by the atmosphere.
I am not sure if I do see things as blue in the dark, I definitely see things as grey whn it is very dim.
The reason that blue is considered cold could be that a blue light is associated with dusk and dawn when it is relatively cold, and red is associated with fires?
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The fact things appear as reddish at sunset it's not surprising, since the color of the sunlight is reddish at sunset!
The fact things appears as bluish with little light is the "Purkinje effect":
http://en.wikipedia.org/wiki/Purkinje_effect
Is due to the fact that, with little light, retinal rods, instead of cones, are active, and they have their max sensitivity for the blue-green, instead of the yellow in the case of cones.
There is a bit of contradiction between the cited Wikipedia article and the one on colour vision, which speaks of three distinct types of cones.
From http://en.wikipedia.org/wiki/Purkinje_effect:
"The Purkinje effect occurs at the transition between primary use of the photopic (cone-based) and scotopic (rod-based) systems: as intensity dims, the rods take over, and before colour disappears completely, it shifts towards the rods' top sensitivity".
I have a book "Light, Colour and Vision" which I read when I was a boy and which has taught me a lot.
About Purkinje effect there is also a picture of houses with yellow walls. The same scene seen with little light, represented in another picture, shows dark, green walls. Colours have shifted toward blue.
Be careful not to confuse this effect with another one: objects seen from very far appears as bluish because of the blue of the sky mixed with the light coming from the object (even during daylight).
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I am not sure if I do see things as blue in the dark, I definitely see things as grey whn it is very dim.
Of course. But when there is little light, but still enough to perceive colours, they are shifted towards blue.The reason that blue is considered cold could be that a blue light is associated with dusk and dawn when it is relatively cold, and red is associated with fires?
Maybe blue comes from water and red from fire.