White light such as sunlight is composed of all colors. When you look through the spectroscope at the bright sky, clouds, or a sidewalk on a sunny day, you see a full spectrum of color. The wavelengths of light in the visible spectrum range from about 400 nanometers (violet) to 750 nm (red).
Green plants appear green because the leaf pigments are reflecting green light. Green light is also transmitted, but it is not absorbed. As you added layers of leaf material, parts of the spectrum were no longer visible, as the plant leaf absorbed them. Blue light is absorbed (not transmitted) when the light passed through only one layer of leaf material. More blue and some red are absorbed by two layers of leaf material. Through three layers of leaf material, only green is transmitted, indicating that both blue and red are absorbed. The most important plant pigment is chlorophyll-a, which exhibits a grass-green color, and absorbs light in the blue and red parts of the spectrum. The absorption of chlorophyll-a peaks at wavelengths of 430 nm (blue) and 662 nm (red). It occurs in all photosynthetic organisms except photosynthetic bacteria. Chlorophyll-b exhibits a blue-green visual color and its absorption peaks at wavelengths of 453 nm (blue) and 642 nm (red). It occurs in all plants, green algae, and some prokaryotes. There is usually about half as much chlorophyll-b as the -a variety in plants.