Light is the energy source for the light-dependent reactions in the thylakoid membranes. Light energy excites electrons in chlorophyll, drives the photolysis of water and is used to make ATP and reduced NADP. These products are then used in the Calvin cycle to reduce GP to GALP.Two features of light matter. Light intensity sets how fast ATP and reduced NADP are made. At low light intensity, the rate of photosynthesis rises roughly in proportion to light intensity, then levels off when another factor becomes limiting. Wavelength matters because chlorophylls absorb mainly red and blue light; green light is mostly reflected, so it drives little photosynthesis.When light intensity falls, less ATP and reduced NADP are available. GP cannot be reduced to GALP, so GP accumulates. Less GALP is made, so less RuBP is regenerated and the amount of RuBP falls.In practical work, light intensity is often varied by moving a lamp towards or away from some pondweed. 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 piece of pondweed. The distance doubles, so light intensity falls to (1/2)2 = 1/4 of its original value.At very low light intensity, the oxygen made by photosynthesis equals the oxygen used in respiration, so there is no net gas exchange. This light intensity is the light compensation point.