Dichromatism
- nicolelyu812
- Feb 25, 2024
- 2 min read
Have you ever observe pumpkin seed oil? What colour is a thin layer of pumpkin seed oil on a white plate? What about a beaker of pumpkin seed oil? Observe their difference in colour. Interestingly, while a thin layer of pumpkin seed oil looks green, a beaker of pumpkin seed oil looks red. This is dichromatism, a phenomenon where the substance’s hue depend on the concentration and the thickness of the medium.
Although William Herschel observed this phenomenon in as early as 1801 when working with aqueous ferrous sulphate and tincture of nutgall, research performed at the University of Ljubljana revealed the answer to the mystery of the colour change.
Conditions for dichromatism
Every substance that has an absorption spectrum with a wide-but-shallow local minimum, and a narrow-but-deep local minimum exhibits dichromatism. And no, it doesn't have to be liquid. Solids works just as well, and I would guess that gases could work too.
Colour that is dependent on thickness....? How? Why?
The colour with the most area above is the colour that is most transmitted. For the shown example, a thin layer of the substance would appear indigo as it has the most area. However, a thick layer of the substance would appear orange. In the "thin" example, orange has a very very low absorbance, thus the absorbance increases much slower than indigo when the substance get thicker.
This phenomenon isn’t solely dependent on the properties of the substance. It is also closely linked to the response of our visual system.
We can also “invert” the absorption spectrum to get the transmittance vs. wavelength graph (to be more intuitive). For the pumpkin seed oil example, the green region is tall (with a high transmittance) and wide, while the red region is even taller but simultaneously narrower. As humans have photoreceptors sensible to green, an apparent green hue is seen when the pumpkin seed oil is of a thin layer below 0.7 mm. When the thickness of the medium increase, the green part of the spectrum - with its slightly lower transmittance - decreases faster than the red part of the spectrum (they are exponentially related), leaving a greater area of the red end of the spectrum. Thus our eyes register red.
Other examples of dichromatic substances include bromophenol blue and resazurin, as well as in some gemstones.
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