When we first learn elementary geometrical optics, the first thing taught to us is the two broad divisions of the type of reflection.-Regular and Irregular or Diffused.
The difference in cause for both of these is that in case of regular reflection an incoming ray is perfectly reflected in a specific direction but in diffused reflection, the reflected ray are diffused in several direction probably owing to minor irregularities of the surface. Hence the diffuse reflection does not form our image as a mirror does but instead produces general luminance. But then in case of multi-media-projectors or when we use a paper or a wall as a screen for real images, an otherwise diffuse surface (wall, paper, etc) produces an image which can be seen by us. Is this not only possible when the incoming rays are REGULARLY reflected on to our eyes?
1 Answer
No, there's no need for screens in the movie theaters to be mirrors i.e. specular reflectors. Quite on the contrary, it's completely necessary for them not to be mirrors i.e. to be diffuse reflectors.
If the screen were a specular reflector, the light would return back into the direction of the projector and would never reach the eyes of the viewers who aren't sitting on the line in which the projector is directed. If the screen were a mirror, the viewers would only see themselves and the projector but couldn't see any magnified versions of the objects that are supposed to be in the movie.
In reality, each point of the screen – which is a diffuse reflector – effectively becomes a source of light whose intensity depends on the amount of incident light at this point and this source is located directly in the plane of the screen. So these sources of light are not images (in the sense of real or virtual images of mirrors or lens) at all. More precisely, the only image of the real "object" – the object on the screen – is formed in the viewers' eyes.
It's important for the projector to sharply illuminate each point of the screen differently, by the correct intensity of light of the right color. This requires precise optics that chooses the right directions of light rays for each point of the movie between the projector and the screen. On the other hand, each point of the screen is a diffuse reflector and much like real objects in the real world, it emits light to all directions so that all viewers may see it, regardless of the location of their chair.
