The directional movement of a plant in response to light is called phototropism. The shoot responds by bending towards light and roots respond by bending away from light.
This happens as follows:
(i) When sunlight falls straight on the plant, the auxin hormone synthesised at the tip of the stem spreads uniformly down the stem and due to equal concentration of auxin, stem grows straight.
(ii) When sunlight fells on only one side of the plant, the auxin diffuses towards the shady side of shoot. The concentration of auxin stimulates the cells to grow longer. Therefore, the stem appears to bend towards the source of light.