Light energy is used in photosynthesis for

Light energy is used in photosynthesis for the Activation of chlorophyll.
Chlorophyll is vital for photosynthesis, which allows plants to absorb energy from light.
Chlorophyll molecules are arranged in and around photosystems that are embedded in the thylakoid membranes of chloroplasts and the function of the vast majority of chlorophyll (up to several hundred molecules per photosystem) is to absorb light.
The function of the reaction centre of chlorophyll is to absorb light energy and transfer it to other parts of the photosystem. The absorbed energy of the photon is transferred to an electron in a process called charge separation.