A mirror does "
specular reflection". Most ordinary objects (trees, grass, sand, concrete, wood, birds, hands, etc) do "
diffuse reflection". As
@Ibix tried to say without using the big words.
In specular reflection, an incident ray is all reflected in a single direction. You know: angle of reflection equals angle of incidence. This is characteristic of smooth surfaces. In terms of wave optics (think
Huygen's principle), the ray path is one that extremizes path length, at least locally. Importantly, this means that the first derivitive of path length with respect to impact location is zero. Small deviations from the center of line result in wave forms that interfere constructively.
By contrast, microscopially rough surfaces tend to reflect diffusely. Small deviations from the center line result in significant variations in path length. There is no constructive interference.
With specular reflection, nearly equal angles of incidence yield nearly equal angles of reflection. A beam of light is all reflected the same way. The angle of reflection yields information on the location of the source.
With diffuse reflection, the angle of reflection tells us nothing about the angle of incidence. The location of the illumination source is not determinable by observing the diffusely reflected light.
We "see" a point on an illuminated object because the portion of the light that is emitted in a very narrow fan-shaped arc stays aligned with the rest of that fan-shaped spray all the way to our eye. The lens of the eye is then able to focus all of that incident light at a single point on the retina that corresponds to the angle of incidence on the eye. This yields what might be seen as a two-dimensional hemispherical bit map of the light incident on the eye.
Post-processing in the eye and the visual cortex mean that the bit map metaphor is not technically apt. But it is good enough for purposes of the ray optics we are discussing.
