Water in a plastic bottle placed on a metal roof can act as a light source due to the phenomenon of refraction and reflection of light. When sunlight hits the bottle, it passes through the plastic bottle and water, bending (refracting) and reflecting back. The plastic bottle can diffuse the light, creating a spread-out lighting effect.
Especially if the plastic bottle is placed on a light-colored roof (like a white metal roof), it helps reflect the light into the water bottle, increasing the lighting effect. Sunlight passing through the water creates an effect similar to a lamp, illuminating the surrounding area.
Additionally, this phenomenon can occur because of the convex shape of the bottle. The shape of the plastic bottle helps concentrate the light and diffuse it outward like a natural light source.