Photosynthesis in nature does not use the far infrared part of the solar spectrum (λ > 900 nm), comprising about 30% of the incoming solar energy. By simple thermodynamic arguments it is explained that this is due to the unavoidable back reactions during the night. It follows that λ ≈ 900 nm provides a natural limit on artificial photosynthesis. The same limit holds for a two-tandem Si solar cell.