I have read this discussion enter link description here , yet I want to make things more clear. In ideal conditions is it true that red laser pointer spot will appear on white sutrface as red while on a green surface it will appear as black point. Let us assume anything needed like let it be an ideal surface, ideal laser pointer etc... Just want to make sure I understnad the main Idea p.s IF needed - let us also assume that there is only one source of light - the red one
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$\begingroup$ This would be a good experiment to do for yourself! $\endgroup$– ptomatoCommented Apr 24 at 3:47
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No, Shining a light on a surface won't make the surface appear darker. If the surface does not reflect any of the light that you shine on it, then it will appear the same as if you were not shining the light at all. You can't make a black dot appear by shining a dot of any color of light onto any surface.
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$\begingroup$ hmm, let us assume that there is only one source of light $\endgroup$– IgorCommented Apr 23 at 15:16
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$\begingroup$ @Igor, It doesn't change my answer. If the surface does not reflect any of the colored light from your laser, then you won't be able to tell whether the laser is on or off by looking at light reflected from the surface. $\endgroup$ Commented Apr 23 at 15:18
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$\begingroup$ So in a dark room ponting a red laser on a green surface will not cause any spot of color appear on the surface like if the surface is not pit at all? $\endgroup$– IgorCommented Apr 23 at 15:49
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$\begingroup$ @Igor, Note: I'm talking about an ideal situation. If the surface is not perfectly matte, or if the surface is not perfectly green, and if your eyes are adapted to the dark room, then you may see a faint red spot when you turn on the laser. Much less bright than what you would see if you shone the laser on a white card. If the surface is only mostly matte and only mostly green, then you may be able to see a red spot on it even in normal, indoor room lighting. $\endgroup$ Commented Apr 23 at 17:17
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$\begingroup$ Given: When I said that the room is dark I wanted to say that there are no any light sources. Given that I do NOT see in the dark till a red laser spot appears (if it appears). So there were 2 scenarios which I wanted to make sure I understood you right: 1 - red laser fall on an ideal white surface - I do see a red spot on it. Right? 2-red laser falls on the green surface - I will not see any spot on the surface. Right? 2.2 If the green surface is not ideal I will see some weak red spot. Right? $\endgroup$– IgorCommented Apr 24 at 6:42