Asked by: Alex Round, London
Opaque sunblocks are effectively paint: particles of white titanium dioxide or zinc oxide suspended in a heavy oil. The oxide particles form an opaque barrier that reflects visible light and ultraviolet. Clear suncreams use organic compounds, such as Phenylbenzimidazole sulfonic acid, that only absorb ultraviolet wavelengths and let visible light through.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.
