The only amino acids that may absorb light are Aromatic amino acids i.e. Tryptophan, Tyrosine, Phenylalanine & Histidine- Most a.a do not absorb visible light & are thus colourless- Aromatic aminoacids absorb ultraviolet light (250- 290 nm).- Tryptophan makes major contribution to ability of most proteins to absorb light in the region of 280 nm.