Reflexion and diffusion

Let's take a look at an even surface (e.g. a piece of aluminium foil or a mirror) that is hit by parallel rays of light. These are reflected on the even surface and leave the mirror as parallel rays of light aswell. Reflexion on even surfaces allows us to see images of other objects (here the observer sees the reflected image of the lamp), the reflecting surface itself is not visible.

If the reflecting surface is rough (e.g. a page of a book or a tree and so on) the imcoming parallel rays of light are reflected in all possible directions. This case is called "diffuse reflexion" or "diffusion". Because of diffusion we are able to see objects that do not glow themselves. Therefore, the observer does not see the reflexion of the lamp but the object itself.

To sum it up:
We are able to see when...

• ...light is emitted by glowing objects and captured by our eyes directly (image above).
• ...light that is emitted by glowing objects is reflected by another object and then captured by our eyes (image below).