Carbon as an atom found in various molecules has unique properties when it comes to heat and to optics. Diamonds are clear and 100% carbon, Coal and graphite are black and mostly carbon. The most thermally conductive materials in the world are made of pure carbon (nanotubes and diamonds). Infrared light is a color of light just like blue and green are but it is too far past "red" for human eyes to see. The sky is blue because blue light gets deflected of off some of the gases in the air and dust particles. Sunsets are red because the sunlight has to go through more air to get to our eyes (skimming the surface of the globe) and all the blue and green light has been removed leaving the red and orange parts of the light. To the color of infrared, the carbon molecule reflects it or you could say it has the color of "infrared" much like the rest of the air has the color of blue. You might say "well that is fine then... it reflects the sun's infrared" BUT the sun only sends some infrared our way and most if it is visible. A unique thing about infrared is that everything produces infrared light. When your stove's electric element gets hot, it starts glowing red. This glow is a small portion of the more intense infrared light coming off that element that we cannot see. Our eyeballs absorb infrared light and thus for snakes to see infrared (so mammals and birds appear to shine to the snake) the snake needs special holes in it's face that focus the infrared light like a pin-hole camera instead of using a lens like normal eyes (which snakes also have but normal snake eyes, like our eyes, cannot conduct that infrared light). The sun shines on the planet, the planet heats up, the planet radiates infrared back out into space which is a good thing... BUT don't forget that the sky is the color of "infrared" when it has CO2 in it (and to a lesser extent, carbon monoxide... it has one oxygen instead of two but still has the same one carbon atom) and to a greater extent other carbon-based molecules like natural gas (made of one carbon atom and 4 hydrogen atoms and written CH4). So visible light comes in but infrared light does not shine back out because the sky is the color of infrared light (if we could see it) and reflecting that infrared light back down at us.
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