The colors of some liquids apparently depend on how much of them you're seeing. According to a recent study in the online edition of Die Naturwissenschaften (The Natural Sciences), the colors of these liquids are also affected by how our eyes perceive color.

The research, performed at the University of Ljubljana in Slovenia, sought to answer the long-standing question of why pumpkin seed oil, traditionally used in eastern Europe as a salad dressing, looks red in a bottle but green on a plate. The answer, it turns out, has to do with the amount of oil you're looking at or, in other words, the oil's depth. "When the layer of oil is less than 0.7 millimeter thick," the study notes, "the oil appears bright green, and in [a] layer thicker than this, it appears bright red."

Researchers were surprised by the findings, because out of the three characteristics of color, two, saturation and brightness (how deep, pale or vivid it is), are normally determined by the thickness and concentration of liquids such as paint, whereas hue (which determines, say, whether the color looks bluish or greenish) is fixed by the material properties of the liquid. For example, blood is red because it contains hemoglobin, and although it may be crimson (if it is pure) or pale red (diluted with water), it will never be green.

White light is composed of different colors that correspond to varying wavelengths. When white light passes through pumpkin seed oil, all of the colors except for a wide range of wavelengths around 520 nanometers (green) and a much narrower range of wavelengths around 650 nanometers (red) are absorbed. But it also soaks up green more strongly than red, which means that the red is brighter, although fewer red wavelengths come out.

When light passes though a thin layer of the oil, a wider range of wavelengths around green are let through, but the intensities of both red and green are only slightly muted because light absorption increases with layer thickness. Coupled with our eyes' higher sensitivity to green (that is, we perceive green to be brighter than an equally bright red), not only do we see more greenish wavelengths (due to the oil) but they also seem brighter. Hence, the oil appears green.

On the other hand, when light passes through a thick layer of oil, a greater amount of green than red wavelengths are still transmitted, but the intensity of green drops much more than that of red, so it is much dimmer. Even our eyes' higher sensitivity to green cannot increase its apparent brilliance to match the red, hence the oil looks red.

Such dichromatism, observed in pumpkin seed oil as well as in the compound bromophenol blue, is very different from other thickness-dependent coloring, like that of soap bubbles and oil films on water. The latter phenomenon is a result of light interference, which causes color to change depending on the viewing angle.