Here's why:

* Light-dependent reactions: The first stage of photosynthesis, called the light-dependent reactions, directly uses light energy to convert light energy into chemical energy in the form of ATP and NADPH.

* Light energy excites electrons: Light photons strike chlorophyll molecules in the chloroplasts, exciting electrons to higher energy levels. These energized electrons are then used to create ATP and NADPH.

While light energy is essential, photosynthesis also indirectly requires other forms of energy:

* Water: Water is a source of electrons and hydrogen ions, which are used in the light-dependent reactions.

* Carbon dioxide: Carbon dioxide provides the carbon atoms needed to build sugars during the light-independent reactions.

In summary: Photosynthesis primarily needs light energy to drive the chemical reactions that convert light into chemical energy in the form of sugars.