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The Law of Reflection and Lasers

  1. Mar 4, 2015 #1
    So I've been looking into the Law of Reflection with Specular and Diffuse Reflection. I understand that electrons have a certain vibrational frequency in objects, and if a light wave matches that, that color wouldn't be observed.

    My question is, what if I took a red laser pointer and shined it at that same object? If it wasn't red in the first place, shouldn't it technically also absorb that red color? I know it's not the case, cause obviously we see that red dot. Does that red laser pointer follow the law of reflection?
  2. jcsd
  3. Mar 4, 2015 #2


    Staff: Mentor

    The law of reflection


    So sunlight has all the colors and the apple absorbs them except for red and so the red is reflected.

    When you shine a red laser on the apple, it simply reflects the red and so you see the red laser dot. The reason you see the dot on a red object is because it may not absorb that specific frequency of red as found in the red laser.
    Last edited: Mar 4, 2015
  4. Mar 5, 2015 #3
    So let's say I have a blue object, which by definition absorbs all wavelengths except blue. If I shine a red laser on it, I still see the red dot. Shouldn't it be absorbing that red wavelength though?
  5. Mar 5, 2015 #4


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    That would require a perfectly blue object.

    Then it's not a perfectly blue object.
  6. Mar 5, 2015 #5
    I think I'm starting to get it. How come though the "not perfectly blue" object only reflects the red light of the red laser pointer but not the sun?
  7. Mar 5, 2015 #6


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    It does reflect some of the sun's red light, but much more blue light, so it appears blue.
  8. Mar 5, 2015 #7
    So when you shine a red laser on it, more red is reflected than blue, so you see a red dot?
  9. Mar 5, 2015 #8


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    Science Advisor

    Yes, this could happen. But to perceive a red dot you don't necessarily need more red than blue light coming from that dot. It might be sufficient to have more red light from the dot, than from the surrounding surface. Our brain calibrates our color perception based on background and ambient light.
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