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Why is visible light invisible?

  1. Apr 5, 2009 #1
  2. jcsd
  3. Apr 5, 2009 #2

    You will see the white light if you are looking at the light source. The white arrow would be shooting into your eye. But if you are looking at the floor, you will see blue, because the blue arrows are shooting into your eye. If there is no air or dust to scatter any of the light, and you look elsewhere (not at the source and not at the floor), you will see black.

    You cannot see a beam of light from the side unless it scatters. Suppose I shoot a laser across the room, it is invisible to you unless I give it some dust so that the light scatters into different directions (which some of them becomes oriented to your eye), or that I shoot the laser directly into your eye. That is why when people use a laser pointer, you don't see a beam of light like a Jedi saber, but you see the dot when it hits the wall.
    Last edited: Apr 5, 2009
  4. Apr 5, 2009 #3
    Thanks that's helpful, but if reflected or incident rays of light are shining directly into our eyes and that allows us to see them, then how does the object/light source appear at a distance? I know two eyes allow us to perceive depth but why is there a space that appears void of something visible if a constant stream of light is reflecting straight into our eyes?
  5. Apr 5, 2009 #4
    You don't need to have two eys to perceive depth. You could have just one eye, but when you move sideway, things that are far appear to be not moving.

    Some animals cannot comprehend a 2D projection of the 3D world. They cannot understand a photographs. It is the human brain that lets you understand depth when you watch TV (a light source of different colors displayed from a flat surface). Some animals would not understand it.

    Actually I don't quite understand what you are asking. :smile:
  6. Apr 5, 2009 #5


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    Staff: Mentor

    What space? As said, if a stream of photons (light) is hitting your eyes, you see them.
  7. Apr 6, 2009 #6


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    Homework Helper

    Because the blue floor absorbs the other colors from the light, only reflecting blue light. If the floor reflect some of the other colors it will appear somewhat as a "blue tinted" mirror. If the light shown at the flow didn't include any "blue" light, such as "red" or "green" light (near monochromatic, or at least not a blend of colors), the blue floor would appear to be black.

    Reflection of light is "subtractive", colors are removed during the reflection process. Light itself, such as a CRT, is additive, the colors are combined and stimulate all 3 or 4 color receptors in the eye (most humans have 3, some have 4), causing the combined colors to appear the same as a single color that would affect the receptors in the same ratio (except that some perceived colors can't be caused by a single frequency of light).

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