Explain what happens to a broken water molecule in the light reaction

The breaking of water by the action of light (photons) during photosynthesis is termed as photolysis.

It takes place in water splitting complex associated with PS II which is located  in thylakoid membrane of chloroplast.

It dissociates water into oxygen, protons, and high energy free electrons. The over all reaction can be written as:

2 H₂O → 4 H⁺ + 4 e⁻ + O₂

The oxygen is released as the byproduct of photosynthesis. It is used in cellular respiration (aerobic) by other organisms.

The electrons released by this process are used to replace the electrons lost from chlorophyll-a through photo-excitation. The electron lost chlorophyll during light reactions are used to reduce NADP⁺ to NADPH which is later used in light-independent phase or Calvin cycle of the photosynthesis.

The protons are released in the thyllakoid lumen. In addition, as electron travels through the ETC, it looses its energy some of which is used to drive pumping of protons from stroma to lumen. All these protons help in production of ATP by the process of chemiosmosis.