Why Does Wet Pavement Look Darker?

Pavement often looks blacker after rain, but the water has not dyed it. A dry road contains tiny grains and pores, with many boundaries between air, mineral particles and bitumen. Light is repeatedly scattered at these rough interfaces, and some of it quickly returns to our eyes as relatively bright diffuse reflection.

When rain fills the pores and covers the surface, many air–solid boundaries become water–solid boundaries. Because water's refractive index is closer to that of many road materials, less light is reflected immediately and more enters the material. Some is absorbed by the bitumen and particles. Light scattered back towards the surface may also undergo total internal reflection at the water–air boundary, sending it into the material again and creating another opportunity for absorption. Less light therefore leaves in the observer's direction, making the road appear darker.

This does not mean that water “absorbs the colour”, or that a wet surface is darker from every angle. A smooth water film produces directional specular reflection, so wet pavement can brightly mirror the sky or headlights at the right angle. The central point is that wetting changes the routes by which light travels and returns. Apparent lightness depends on how much light finally reaches the eye, not only on the material's original colour.

https://opg.optica.org/ao/abstract.cfm?uri=ao-27-7-1278
https://ece.ncsu.edu/2024/why-do-things-look-darker-when-theyre-wet/