by Ann » Wed Feb 08, 2012 1:51 am
Chris, I don't doubt for a moment that various evolutionary processes have led to the color vision that humans have today. I still believe that our three-color vision, where green is a primary color, has had an important survival value for humanity.
But there is another reason why we don't ever see the sky as green (apart from auroras, which is another matter). The "transition sky", which is neither red, yellow nor blue, must be similar enough to the "unfiltered" light of the Sun to appear to be the same color of the same, namely white.
Consider. Why is it that we
never see stars as green? It is because their light is similar enough to the light of the Sun that they appear white to us. Stars whose light output peaks in the orange or red part of the spectrum look (mildly) yellow. Stars whose light output peaks in the infrared part of the spectrum look yellow, yellow-orange, or, in extremely rare cases (Mu Cephei!) orangeish. Stars which have "polluted" their own atmospheres with large amounts of carbon, so called carbon stars, may block so much of their own shorter wavelengths that the stars in fact look red. I have seen only one such star, V Aquilae.
Stars which are much hotter than the Sun can indeed look bluish, although their color is never saturated, and very few people can detect their color without a telescope. If you have a telescope and look at a hot and relatively unreddened star, however, you can often detect a bluish cast to its hue.
But stars
never look green.
The Sun can be described as a green star, since its energy output peaks in the green part of the spectrum. But we don't see sunlight as green. We see it as white. On a mildly overcast day, when we can't see the blue of the sky, daylight is neither yellowish nor greenish. The color of daylight is still white, or a neutral gray.
Freshly fallen snow, which reflects most of the light that hits it equally, never looks green, even though it may reflect more green light than any other color. Snow either looks white to us, or, on a sunny day, yellow-white in the sun and blue in the shadows. (Or else it looks dirty and brownish.) It just never looks green. If you see a spot of green in the snow, it means some grass is peeking through.
Green auroras have an extremely strong peak in the green or yellow-green part of the spectrum, which is why we can easily see their color. (In fact, they may contain only green light.) But it is impossible for us to see with our own eyes that the light of the Sun peaks in the green part of the spectrum, so that sunlight and daylight is "really" green. It never looks that way to us. And no part of the twilight sky looks green, either.
Ann
Chris, I don't doubt for a moment that various evolutionary processes have led to the color vision that humans have today. I still believe that our three-color vision, where green is a primary color, has had an important survival value for humanity.
But there is another reason why we don't ever see the sky as green (apart from auroras, which is another matter). The "transition sky", which is neither red, yellow nor blue, must be similar enough to the "unfiltered" light of the Sun to appear to be the same color of the same, namely white.
Consider. Why is it that we [i]never[/i] see stars as green? It is because their light is similar enough to the light of the Sun that they appear white to us. Stars whose light output peaks in the orange or red part of the spectrum look (mildly) yellow. Stars whose light output peaks in the infrared part of the spectrum look yellow, yellow-orange, or, in extremely rare cases (Mu Cephei!) orangeish. Stars which have "polluted" their own atmospheres with large amounts of carbon, so called carbon stars, may block so much of their own shorter wavelengths that the stars in fact look red. I have seen only one such star, V Aquilae.
Stars which are much hotter than the Sun can indeed look bluish, although their color is never saturated, and very few people can detect their color without a telescope. If you have a telescope and look at a hot and relatively unreddened star, however, you can often detect a bluish cast to its hue.
But stars [i]never[/i] look green.
[float=right][img]http://www.diycalculator.com/imgs/console-09.gif[/img][/float]The Sun can be described as a green star, since its energy output peaks in the green part of the spectrum. But we don't see sunlight as green. We see it as white. On a mildly overcast day, when we can't see the blue of the sky, daylight is neither yellowish nor greenish. The color of daylight is still white, or a neutral gray.
Freshly fallen snow, which reflects most of the light that hits it equally, never looks green, even though it may reflect more green light than any other color. Snow either looks white to us, or, on a sunny day, yellow-white in the sun and blue in the shadows. (Or else it looks dirty and brownish.) It just never looks green. If you see a spot of green in the snow, it means some grass is peeking through.
Green auroras have an extremely strong peak in the green or yellow-green part of the spectrum, which is why we can easily see their color. (In fact, they may contain only green light.) But it is impossible for us to see with our own eyes that the light of the Sun peaks in the green part of the spectrum, so that sunlight and daylight is "really" green. It never looks that way to us. And no part of the twilight sky looks green, either.
Ann