How does phototropism occur in plants?

Phototropism are curvature movements that occur when a plant is provided with artificial, natural light just from one direction. Plant parts that show a bending towards the source of light are said to be positively phototropic, whereas plant parts that bend/ grow away from light source are said to be negatively phototropic.

This curvature / bending of parts usually stem is due to unequal distribution of auxin under the influence of light. Auxins accumulate on side away from light. So wherever auxin concentration will be high, the stem will promote growth on that side.