Graphical techniques are easily learned and tend to be remembered unlike complex mathematical representations. These rules must be consistent with the previous labs but will hopefully provide us with additional information. We see people and things because light is reflected off of them in our direction. The same is true for everybody else. This effect is known as diffuse reflection.
Rough objects are really just very bumpy mirrors at microscopic level.
As a result many people share in the ability to see the surface simultaneously. Lenses and curved mirrors do different things to light. One may think of them as processing light to produce some desired effect. It is not always true that all observers share in the effect being created. Only the film sees the effect created by the camera lens.