Light provides the energy for the light-dependent reaction. Light excites electrons in chlorophyll, and this drives the photolysis of water and the production of ATP and reduced NADP. These are then used in the light-independent reaction to reduce GP to triose phosphate.At low light intensity, too little ATP and reduced NADP is made, so light limits the rate of photosynthesis. Over this range the rate rises roughly in proportion to light intensity. At higher intensities the curve levels off, because carbon dioxide concentration or temperature is now limiting.In very dim light, the rate of photosynthesis is lower than the rate of respiration, so the plant gives out carbon dioxide overall. The light intensity at which the two rates are equal is the light compensation point.
In experiments, light intensity is often varied by moving a lamp. Light intensity is inversely proportional to the square of the distance from the source:light intensity ∝ 1 / d2 Worked example: A lamp is moved from 10 cm to 20 cm from a plant. The distance has doubled, so the light intensity becomes 1 / 22 = 1/4 of its original value. Moving the lamp from 20 cm back to 10 cm makes the light 4 times as intense.Wavelength matters too: chlorophyll absorbs mainly red and blue light and reflects most green light, so green light gives a low rate of photosynthesis.
In experiments, light intensity is often varied by moving a lamp. Light intensity is inversely proportional to the square of the distance from the source: