White light passing through a piece of green glass emerges as green light. A possible explanation would be that the green glass adds “greenness” to the white light, but Newton found otherwise. Green glass is green because it absorbs any light that is “not green.” We can think of a piece of colored glass or plastic as a filter that removes all wavelengths except a chosen few.

If a green filter and a red filter are overlapped, as in FIGURE 35.20, no light gets through. The green filter transmits only green light, which is then absorbed by the red filter because it is “not red.”

FIGURE 35.20 No light at all passes through both a green and a red filter.

This behavior is true not just for glass filters, which transmit light, but for pigments that absorb light of some wavelengths but reflect light at other wavelengths. For example, red paint contains pigments reflecting light at wavelengths near 650 nm while absorbing all other wavelengths. Pigments in paints, inks, and natural objects are responsible for most of the color we observe in the world, from the red of lipstick to the blue of a blueberry.

As an example, FIGURE 35.21 shows the absorption curve of chlorophyll. Chlorophyll is essential for photosynthesis in green plants. The chemical reactions of photosynthesis are able to use red light and blue/violet light, thus chlorophyll absorbs red light and blue/violet light from sunlight and puts it to use. But green and yellow light are not absorbed. Instead, to conserve energy, these wavelengths are mostly reflected to give the object a greenish-yellow color. When you look at the green leaves on a tree, you’re seeing the light that was reflected because it wasn’t needed for photosynthesis.

FIGURE 35.21 The absorption curve of chlorophyll.