We all have fond memories of playing the buttercup game. You simply hold a buttercup flower underneath your chin, and if your neck lights up yellow then it's conclusive, empirical proof that you like butter.

But physicists at the University of Cambridge have discovered the real reason that these flowers give off that golden glow. Disappointingly, it's more about the unique interplay of the petals' epidermal layers, and less about your fondness for margarine.

To make that gorgeous yellow hue, the petals of the buttercup flower (Ranunculus repens) have a carotenoid pigment which absorbs light in the blue and green region of the optical spectrum. The result is that the other colours -- in this case, mostly yellow -- are reflected back.


The light passes through to the epidermal layer of the petals.

This is a layer of flat cells, from which light is reflected. On the buttercup, though, this layer has not one but two extremely flat surfaces, separated by a gap of air.

Reflection of light by the smooth surface of the cells and by the air layer effectively doubles the gloss of the petal. This causes the buttercup to reflect light under your chin better than any other flower on the school sports field.


In their research, the Cambridge flower-pickers found that the buttercup's petals are also good at reflecting ultraviolet light.

As many pollinators, such as bees, have eyes that are sensitive in the UV region, this could explain how the buttercup uses its unique epidermal layers to attract insects.

Plants have an arsenal of different techniques to attract pollinators, including scent, temperature and of course visual identifiers. "Flowers develop brilliant colour, or additional cues, such as glossiness -- in the case of the buttercup -- that contribute to make the optical response of the flower unique," explains Silvia Vignolini, from the University of Cambridge's Department of Physics. "Moreover, the glossiness might also mimic the presence of nectar droplets on the petals, making them that much more attractive."