|Dispersion of visible light by a prism|
|Portions of the spectrum including the infrared and visible portions.|
Note 4. Why is CO2 a greenhouse gas? Most molecules behave as if they have a “color” in the part of the unseen light spectrum that transmits heat. This is called the infrared spectrum (see the graphic in Note 2). Different molecules have different “colors” in the infrared part of the spectrum, just like different substances having different colors to our eyes in the visible spectrum (such as the chlorophylls and carotenoids in the graphic in Note 1). Atmospheric CO2, while permitting the visible light of sunlight to pass unimpeded through the air to the ground, does have an infrared “color”, and partially absorbs heat radiated back into the atmosphere from the surface of the earth after colored objects have absorbed sunlight (see the graphic below; here absorbed infrared radiation is indicated in the downward troughs, and transmitted or unabsorbed light is shown by the flat portions at the top).