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Basic Question About How Matter Absorbs Light

  1. May 10, 2010 #1
    I'm not a physicist. So mathematics probably wont help me understand this issue. However, this has been troubling me for the past week or so and I would really like to get to the bottom of it.

    If someone could provide a plain English explanation of what's going on I would greatly appreciate it.

    If I have a red wall (no gloss, just a dull red) and I shine a white line on the wall I get back a brighter shade of red where the light illuminates the wall.

    If I increase the intensity of the light source I will eventually see white light being reflected off this red surface.

    Why does this happen?

    My intuition, obviously misguided, tells me that I should be getting a neon red back from the wall. In other words the red surface will keep absorbing all wavelengths of light except red up until the point where the heat generated burns the surface.

    What instead happens is the surface appears to lose its ability to absorb non-red wavelengths as the light intensity is increased and I slowly get more and more white light.

    This confuses me because I can't find any literature describing this phenomenon. Everything I can find simply says that light is either absorbed or reflected, but nothing about an absorption limit.
    Last edited: May 10, 2010
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  3. May 10, 2010 #2

    Andy Resnick

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    Without any specifics (i.e. why is the wall red, and what is your light source), it's hard to say.

    But, if you wall is red because of paint, or some absorptive pigment (colored glass/plastic), then the wall actually preferentially reflects red- this explains the first observation.

    As to the second, again it's hard to say without knowing what your source is, but most likely what's happening is the remainder of light not absorbed is sufficiently bright for your eyes to see, leading to color mixing etc., leading to a white appearance.

    There's no saturation/absorption limit involved.
  4. May 10, 2010 #3
    Hmmm. That's a pretty good question.
    But when you say "dull" red, I suppose there might still be some amount of "gloss" that reflects the white light. This would be more noticeable at higher light intensities.
    Just my guess, though.
  5. May 10, 2010 #4
    I think where you're misunderstanding lies is that objects aren't perfect absorbers/reflectors. If an object were to reflect at, and only at, a wavelength of 650 nm (red) then yes, it would not look white if white light were shined on it. However, a red object predominantly reflects red light, but also will reflect other colors in smaller amounts. When you shine brighter and brighter white light you reflect more and more other colors which will then seem white (note that white is hard to define, there are shades of white so to speak).

    A quick example would be shining a red laser pointer at a black wall. If it were perfectly black, you would see no red dot, but in real life you do see that something black will in fact reflect some light - just not as much as a white object, on which the red laser dot would appear much brighter.
  6. May 10, 2010 #5
    Thanks for the replies!

    Sorry for not being more specific. Yes, I am assuming it is red because of paint, and the light source would be, lets say, a search light with a intensity control on it so I could turn up and down the brightness.

    So what you are saying is that most surfaces don't absorb 100% of wavelengths of light. Instead my red well is likely just absorbing most, or a large percentage, of the incoming non-red light and bouncing back a high percentage of red light.

    When I increase the intensity the previously unnoticeable amount of non-red light suddenly becomes more apparent and so I see white light.

    I suppose what I don't understand is that since there will always be more red light than white light wouldn't I be seeing pink? Sure there might be more white light but at the same time the red light would MUCH stronger as well.

    The ratio of red to white light would always result in more red light than white...
  7. May 10, 2010 #6
    That's what I meant by "shades of white." Incandescent lightbulbs output a lot more red light than anything else, but they still look "white." A lot of the time you need to compare side by side different white lights to really tell the difference. You are correct, though, that there will always be more red light reflecting in your proposed case, and as such it should look more red than if it where shined on a white surface. Another thing that comes to mind is the human eye - that can become saturated when the light intensity gets higher.
  8. May 10, 2010 #7
    That's good point. So does the human eye precieve all light of a high intensity, regardless of wavelength, as white?
    Last edited: May 10, 2010
  9. May 10, 2010 #8

    Andy Resnick

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    Ah! That's a central feature of how color vision works- after all, there's no 'pink' wavelength, just as there's no 'purple'.

    Your brain creates color, not your cone receptors.
  10. May 10, 2010 #9
    I see, it makes a lot of sense this might be an issue with color perception.

    However, this doesn't make sense though, because lasers operate at VERY high intensities but appear to be only one color...

    Is it the case then that my eyes get over saturated by the red light, stop seeing it regardless of an increase of intensity, which then results in a precived increase of white light?

    Or am I barking up the wrong tree?
  11. May 10, 2010 #10
    Definitely not.
  12. May 10, 2010 #11
    Yeah I didnt think so, but it might make sense about the saturation limit.

    I guess the real issue I have is that if someone shines a red lazer into your eye you have the perception of being blinded by red light.

    However, if I shine a white light at a red wall and increase the intensity enough I will eventually have the perception of being blinded by WHITE light and not red...

    So why is the wall any different than the lazer?
  13. May 10, 2010 #12

    Andy Resnick

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    Again, it's the source- lasers are nearly monochromatic, searchlights/halogens/LEDs etc. are not.
  14. May 10, 2010 #13
    Thanks for consistent replies Andy.

    I'm probably just missing something here, but I'm still a bit confused.

    I understand that the laser is monochromatic and that other light sources are not, but what confuses me is the order in which things occur.

    In my head, I'm imagining being blinded by white light before I am ever blinded by red light. I understand that since the light source isn't pure red, and that because the surface does reflect some white light, I will EVENTUALLY be blinded by white light, but before that happens I would imagine I would have an experience much like I would if someone shined a red laser in my eye.

    Is this correct, or am I mistaken?

    In other words I would be overwhelmed by red light before I was by white light, but that doesn't appear to the the case, or at least when I imagine it. It seams like the red light gets bright to a point and then stops at which point I start seeing white.
  15. May 11, 2010 #14

    Andy Resnick

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    I'm not exactly sure what you are asking now.
  16. May 11, 2010 #15


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    Like pallidin stated, no. However, we are fairly tolerant to what we perceive as white. Photography can make this blatantly apparent with white balance. If you were to view a piece of white paper in a room lit by an incadescent light, flourescent light, and sunlight you would probably feel that in each of the three circumstances the paper appears white. However, if you were to take a picture of it with a camera in each of the three situations you would find a definite red/yellow and blue tint to the white for the incandescent and flourescent lighting respectively. White balance is an annoying little bugger to correct because of this.
  17. May 11, 2010 #16
    After researching white blance I think this might have to do with color tempature. The color of the light IS changing the more intense I make it.

    My light source most likely mimics a http://en.wikipedia.org/wiki/Black_body" [Broken]. So you actually get a more pure white light the more intense you make it.

    Just imagine a black stone in a fire. It starts off red and then goes through several other color changes before it eventually starts to produce white light.

    So how "dirty" a white light is has a direct correlation with how intense it is.

    In other words the way the wall reflects and absorbs light ALWAYS stays the same, but the color of the incident light does not. I just needed to flip around my thinking on this.

    Does this sound correct?

    Here is more on color temperature:

    http://www.cambridgeincolour.com/tutorials/white-balance.htm" [Broken]
    Last edited by a moderator: May 4, 2017
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