Dichromatism

Dichromatism (or polychromatism) is a phenomenon where a material or solution's hue is dependent on both the concentration of the absorbing substance and the depth or thickness of the medium traversed.

The phenomenon is related to both the physical chemistry properties of the substance and the physiological response of the human visual system to colour.

[3] Dichromatic properties can be explained by the Beer–Lambert law and by the excitation characteristics of the three types of cone photoreceptors in the human retina.

This means that pumpkin oil changes its colour from green-yellow to orange-red (for 44 degrees in Lab colour space) when the thickness of the observed layer is increased from cca 0.5 mm to 2 mm; and it changes slightly towards green (for 9 degrees) if its thickness is reduced for 4-fold.

A record by William Herschel (1738–1822), shows he observed dichromatism with a solution of ferrous sulphate and tincture of nutgall in 1801 when working on an early solar telescope, but he did not recognise the effect.