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Today, me and my brother looked at red point of the wall (that was a red sticker for kids), and we saw the red point even we changed our locations in the room, and I can argue that everyone can see it at any point of the room.

Simply, as I am not professional in this subject but I just interested to find out how things work. So, I think that the lights that hit that point will reflect and then go for each directions, but if we consider infinite people in the room, dose everyone see the red dot? How even a low light source can be seen from each direction and from far distance?

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When light hits an opaque object it gets reflected. The direction in which it bounces back depends on the nature of the surface (material, texture...) and the direction of the incoming rays. Generally speaking the reflected light can be:

  • specular, like a mirror, the outgoing rays will be more or less on the opposite angle of the incident ray, just like in a mirror.
  • diffuse, the outgoing rays will be spread out in all directions.

Also incident light can also be absorbed by the object instead of reflected.

Of course real objects always reflect light partly specular and partly diffuse.

Now, you seem to think that since light comes in infinitely thin rays from one direction and reflects to infinite directions, so the intensity of the rays should dim to zero intensity.

But that is wrong, you should think instead in terms of light intensity per surface unit, that is luminance.

If you think carefully, light sources have a non-zero surface, the red object have a non-zero surface, and your eyes also have a non-zero surface (actually your pupils). There are no zeros and no infinites in this problem ( cannot put an infinite number of eyes in a room!)

Instead of rays of light try to think in terms of luminous energy and luminance and you'll see that the problem you are asking is actually not so difficult.

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