Light provides energy for the synthesis of glucose from carbon dioxide and water during photosynthesis.
Photosynthesis is a photo-chemical reaction involving 2 main steps, i.e. Light reaction or Hill’s reaction and Dark reaction or Blackmann’s reaction.
Light reaction occurs in presence of light. Dark reaction can occur in absence of light but is dependent upon the end product of light reaction. Thus light reaction must precede dark reaction.
During light reaction, the chlorophyll entraps light and the solar energy is converted into chemical energy in the form of ATP molecules. This can happen because light energy is used to split water. The products of this reaction are oxygen (yeah for us heterotrophs!) and hydrogen ions. The hydrogen ions are used to make the ATP mentioned earlier.
Here is a video showing the formation of oxygen in the light reaction.
Because it is the part where glucose is produced. The light dependent part has the function of fixing the light energy from the sun in the form of ATP molecule.This ATP will be used in the light independent part to make the high energy bonds in the glucose molecule.