phototropy

/ (ˌfəʊtəʊˈtrəʊpɪ) /

noun chem

an alteration in the colour of certain substances as a result of being exposed to light of different wavelengths
the reversible loss of colour of certain dyestuffs when illuminated at a particular wavelength

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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012