Here's why:

* Photosynthesis is the process where plants convert light energy from the sun into chemical energy in the form of sugars.

* Chlorophyll, the green pigment in plants, absorbs light energy, primarily in the red and blue wavelengths.

* This absorbed light energy is then used to split water molecules and power the production of glucose, the plant's food source.

So, while plants utilize other forms of energy like chemical energy from nutrients in the soil, the primary energy source for photosynthesis is light energy.