Dry sand appears bright because it reflects more light, while wet sand appears darker due to the optical properties influenced by water. When sand is dry, light reflects off the individual sand grains, causing the sand to appear bright and shiny. When sand becomes wet, water fills the gaps between the sand grains, changing the way light interacts with the surface. The water reduces the amount of light reflected directly and increases the amount of light absorbed or refracted within the sand-water mixture. This results in less light being reflected back to the observer, making the wet sand appear darker and less bright. Therefore, the change in appearance is caused by refraction (C), where the water alters the light paths through the sand, reducing its overall brightness compared to dry sand.