EM radiation impinging a rough surface is (partly) reflected back (scattering). Lambertian surfaces produce a diffuse scattering (i.e. radiation is reflected similarly in all direction) and then appear equally bright from all directions, whereas specular surfaces behave like a mirror, with reflected radiation all aligned in one direction, with the reflection zenith angle equal to the incident angle of incoming radiation. Generally, the degree of "roughness" of a surface determines if it behaves like a Lambertian or a specular surface.