According to the principles of photosynthesis discussed in the NCERT text, the discovery of two discrete photochemical light harvesting complexes (LHC) within Photosystem I (PS I) and Photosystem II (PS II) was pivotal. Historically, Emerson's experiments showed that photosynthetic yield drops at wavelengths beyond 680 nm ('Red Drop') but enhances when both far-red and shorter wavelength red light are given together ('Enhancement Effect'). This confirmed the existence of two photosystems (PS I and PS II) operating simultaneously/in series during the light reaction.