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Homework Help: Simple question about laser light

  1. Jul 1, 2007 #1
    Nobody has been able to give me a solid answer to my question:

    If you had a red, green and blue laser, and you combine the three beams to one spot will you see a white spot?
    Since lasers are coherent the three beams should pass through each other without forming a white spot. Right? Or do our eyes perceive them to be white?

    What if all three beams were coherent with one another ie. synchronous phase between red,blue & green, will you still see a white spot?
  2. jcsd
  3. Jul 1, 2007 #2


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    You must have the correct wavelenght to get that effect I belive. Red is not red, but just one type of red.
  4. Jul 1, 2007 #3
    I don't think a "particular" red is needed. Anything that emits a particular wavelength of light has a spectral width.

    With black body radiators using a red, blue and green coloured filament bulbs gives white (almost white) when overlapped because their spectral bandwidth is extremely large so they effectively have a 'rainbow' of colours.
  5. Jul 8, 2007 #4
    I am not that big on biology, but I believe to perceive white light you only need to stimulate the three types of cones in your eye in roughly equal proportions, so I think red, green, and blue lasers would do the job, regardless of coherence or bandwidth.
  6. Jul 8, 2007 #5


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    I think your confusion may have more to do with how we see light.

    "...the three beams should pass through each other..."

    This makes no sense. The laser light needs to impinge upon a surface and reflect back into our eyes.

    If we see a spot that is emitting light that stimulates our red green and blue receptors, then yes,. it should appear white.
  7. Jul 8, 2007 #6
    IIRC the eye is more sensitive to yellow-green light, so the receptors would have to receive the correct proportions of light of the three colours to see white as pure white. It's a bit complicated if I'm remembering correctly but essentially they use intensities to map the colour spectrum mathematically.
  8. Jul 8, 2007 #7
    Somewhat leaving the topic now, but would it be correct to say that you cannot visually distinguish lasers of different wavelengths (considering sufficiently small bandwidths) as long as they stimulate the same receptor, i.e. any "red" laser is perceived with no more detail than being purely "red"?

    The wavelength cutoffs for each receptor are however almost certainly half-maximums or something similar, so there may be enough overlap for this not to be the case.
    Last edited: Jul 8, 2007
  9. Jul 9, 2007 #8

    If you had the right combinations (intensities) and if you shined them all the an opaque diffuse* surface like a white piece of paper and assuming they were nearly colinear then you should see white.

    *Once it is reflected off an opaque diffuse surface the coherence is destroyed (or significantly reduced).

    You are correct in your assumption that coherent light doesn't mix (in vacuum). It needs to be carefully engineered to produce white (I'm assuming a loose definition of white) light. Although it is possible.
    Last edited: Jul 9, 2007
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