Explanation

Development of proton gradient across the membrane. During the cyclic phosphorylation, the electrons does not pass to the $NADP^{+}$ but cycled back to PS-I through electron transport chain. This cyclic flow hence, results only in the synthesis of ATP but no of NADPH + $H^{+}$. Cyclic photophosphorylation occurs when only light of wavelength beyond 700 nm is available for excitation