Question

In which of the following wavelengths, photosystem-I is inactive?

• A. 780 nm
• B. 680 nm
• C. 690 nm
• D. 550 nm

Answer 1

The correct option is A 780 nm

In photosystem-I, the photo centre is a molecule called $P_{700}$. The P700 reaction centre is composed of modified chlorophyll a that best absorbs light at a wavelength of 700 nm, or less. Higher wavelengths are ineffective and lack the energy required for activation of the photo centre. This is called red drop.
Therefore, at a wavelength of 780 nm, photosystem-I is inactive.