All right, so I’ve got a pair of sunglasses with red lenses. They’re very cool, cause they make the sky look purple and every red thing look bright orange.
But I got to wondering this afternoon. What color would the sky have to be in order to appear green behind my lenses? If I remember my art classes correctly, that can’t happen. However, this isn’t art; it’s physics. So what’s up?
I don’t think it’s possible. In theory, everything should appear as different shades of red, as the sunglasses block all non-red light. However, nobody makes sunglasses that block all non-red light, because that would be useless. So they probably settle for blocking most other colors, increasing the blockage the further the color is from red. Thus, the light that gets through is dimmed in all colors aside from red.
The light blue of the sky could be divided into white and blue light. The white light contains all colors more or less equally, so when it passes through the red lens, all colors are dimmed except for the red. The blue component of the sky is simply dimmed to darker blue. Thus, you see purple.
The only way to get green would probably be to look at bright green light. Since the red in question is probably more of a red-orange, green would be the color most dimmed by the lenses, and thus the hardest to get.
I once tried on a pair of dark red glasses when I was at a campfire, and was amazed to see bright green flames at the center of the fire! These flames only appeared at the hottest point of the campfire near the coals, not the yellow flames above. Others at the campfire saw the same thing, but no one could explain why. And, no, we weren’t high on any psycho-active substances (at least, none stronger than a couple of beers). Any ideas?
The flames at the center of the fire probably were green, which happens at the correct temperature (hotter than yellow, colder than blue, I think). With the red lenses, the red flames wouldn’t have been as noticeable, like when you use the red film to read secret messages on cereal boxes and stuff, where the red scribbles hide a message in blue.
Ideally and simplistically, with sunglasses that transmit only red light and eyes that divide light into purely red, green and blue components, everything you see through the sunglasses will appear red and nothing but.
However your eyes have overlapping spectral responses in their red, green and blue sensors. In particular the red and green sensors overlap by a great deal. Furthermore, most color filters (eg colored eyeglasses) have pretty gradual cutoffs of wavelengths further and further from those they are intended to transmit. The combined effects of these and other problems (such as some color receptors growing tired after viewing strong colors) let you see some variety of colors you might not expect to.
BTW there isn’t any temperature that makes things glow green with true blackbody radiation. The light emitted by heated bodies is too broadly distributed over the spectrum to concentrate in our relatively narrower green-sensitive range.