Chlorophyll is the principal pigment involved in photosynthesis. Chlorophyll ' $a$ ' is the major pigment involved in trapping light energy and converting it into electrical and chemical energy. Chlorophyll ' $b$ ' molecules also acts as accessory pigment. Other accessory pigments are phycoerythrin, phycocyanin, carotenoids etc. The accessory pigment and reaction centre together form photo-system.