Diffuse reflection and everyday objects

Two types of reflection

Not all surfaces reflect light in the same way. There are two distinct cases:

• Specular reflection: on a smooth surface (a mirror, still water, polished glass), parallel rays remain parallel after reflection. This produces a clear image.
• Diffuse reflection: on a rough surface (paper, a wall, fabric, a road), each tiny bump reflects the light in a different direction. The rays scatter in all directions, without forming a clear image.

Why can we see objects that do not shine?

It is thanks to diffuse reflection that we can see an object from anywhere in the room: the light it receives is scattered in all directions, and some of it always reaches our eyes, regardless of our position.

Comparison table

Surface type	Type of reflection	Example
Smooth and polished	Specular	Mirror, glass, still water
Rough	Diffuse	Paper, wall, dry road

Practical example

A wet road becomes dangerous at night because the water smooths the surface: the reflection becomes more specular, and the headlights dazzle rather than illuminating the road surface as they normally would, which in dry conditions reflects light diffusely.

Common pitfall

Do not assume that a non-luminous object does not obey the laws of reflection: even a rough surface obeys i = r at the level of each micro-facet; it is only the orientation of these facets that varies, causing the light to scatter.