"In vivo" chlorophyll "a" fluorescence is a key technique in photosynthesis research. The recent release of a low cost, commercial, modulated fluorometer enables this powerful technology to be used in education. Modulated chlorophyll a fluorescence measurement "in vivo" is here proposed as a tool to demonstrate basic photosynthesis phenomena to students of Years 11 and 12 in secondary education. These phenomena are already part of the Portuguese Biology syllabus and include: light absorption by photosystems; primary charge separation at the reaction centre of photosystem II; electron transport in the Z-scheme; energy transduction; and integration between photochemistry and the Calvin-Benson cycle. (Contains 1 table and 6 figures.)