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Thread: reflected v emitted light

  1. #1 reflected v emitted light 
    SHF
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    My background is in biology rather than physics so perhaps this is a question with a very simple answer.

    Why can we distinguish reflected light from emitted light (if those are the right terms). What I mean is that the 'red' from a red LED (or whatever) looks rather different from the 'red' from a red table. What is the nature of the difference? (something to do with wavelengths? intensities?)


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    Brassica oleracea Strange's Avatar
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    It is probably to do with the angular distribution. An LED, for example, is close to a point source; very bright where the LED is and no light elsewhere. A reflection from a table will be diffuse; a fairly even source, perhaps slightly brighter in the direction of the light illuminating it.

    If you see a very shiny curved surface then you may get a small highlight which could look like there was a light inside the bject. Conversely, you could have a lamp with a diffuser that might just look as if it is a brigtly lit surface.


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    is polarisation anything to do with it? reflected light is polarised, not sure if always though.
    Sometimes it is better not knowing than having an answer that may be wrong.
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Chrispen Evan View Post
    is polarisation anything to do with it? reflected light is polarised, not sure if always though.
    Interesting point. I don't think the human eye can detect polarization but it might have some subtle effect.

    Edit: Note that only reflection from non-metallic surfaces is polarized.
    Last edited by Strange; October 22nd, 2012 at 04:34 AM.
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    Quote Originally Posted by SHF View Post
    Why can we distinguish reflected light from emitted light (if those are the right terms). What I mean is that the 'red' from a red LED (or whatever) looks rather different from the 'red' from a red table. What is the nature of the difference? (something to do with wavelengths? intensities?)
    In this case it's merely intensity and scattering. If I replaced the table with a mirror, you wouldn't be able to tell the difference between the emitted light and the reflected light.
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    Forum Freshman pogomutt's Avatar
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    Quote Originally Posted by Farsight View Post
    Quote Originally Posted by SHF View Post
    Why can we distinguish reflected light from emitted light (if those are the right terms). What I mean is that the 'red' from a red LED (or whatever) looks rather different from the 'red' from a red table. What is the nature of the difference? (something to do with wavelengths? intensities?)
    In this case it's merely intensity and scattering. If I replaced the table with a mirror, you wouldn't be able to tell the difference between the emitted light and the reflected light.
    Maybe, maybe not. It would depend on the wave front of the mirror and the acuity of the eyes looking upon the reflection. In most cases I'd say you're correct however, because the intensity of a bright LED and its close proximity to the mirror, is going to mask aberration, coma, fringing, etc. in the reflected image.

    Most of astronomy is about mirrors reflecting point sources of light. The holy grail of telescope making is to create an instrument whose reflected images are identical to the point source of light. Most of the world's great telescopes (including Hubble) are Newtonians or some catadioptric variation of Newtonians. In a Newtonian telescope, the objective mirror should have a 1/4 wave front or better to eliminate most of the aberrations inherent in reflected light. Good telescopes can do that, but the intensity of reflected light will always be a fraction less than naked eye observations of a point source, assuming the object can be seen with the naked eye in the first place. New color-correction and jitter elimination software developed by the Lawrence Livermore lab can do marvels as well.
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    Fair enough pogomutt. There's the polarization point too. I was thinking of the standard Mark I Eyeball.
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    edd
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Chrispen Evan View Post
    is polarisation anything to do with it? reflected light is polarised, not sure if always though.
    Interesting point. I don't think the human eye can detect polarization but it might have some subtle effect.

    Edit: Note that only reflection from non-metallic surfaces is polarized.
    The eye can just detect polarisation in some circumstances, but I don't think this is one of them (I've never managed it myself, but it is well documented - Google 'Haidinger's Brush').
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    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by Strange View Post
    Edit: Note that only reflection from non-metallic surfaces is polarized.
    Interesting. I wasn't aware of this small but important bit of trivia.
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by edd View Post
    The eye can just detect polarisation in some circumstances, but I don't think this is one of them (I've never managed it myself, but it is well documented - Google 'Haidinger's Brush').
    Fascinating. I'll have to try it. (Next time we have a clear sky ... which may be some time.)
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    SHF
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    Thanks for the information everyone. To be honest I don't completely understand the answers - purely because I have no physics so have to look up even basic terms and concepts, but this has helped direct my gaze toward the right topics to read around so that I can understand your answers better. Merci beaucoup
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    In general a white surface will have random polarization (though specialised surfaces do exist for this, I actually remember there was something quite recently about this). And emitted light will have a less random phase. Also the focusation is indeed different. But light is light. The main difference is probably the de-coherence length.
    Artificial sources have a larger decoherence length then more pure ones like reflection and the sun or a wolfram filament. Also, metallic-ism hasn't really all that much to do with polarization. It has more to do with crystalization. The more ordered a material is, the more likely it is prown to have a certain polarization preference. (hence liquid crystal display)

    But to refer from your original question. I think it is intensities. If I have a bright light, and a laser spot in the same colour. It will both seem as a light source. In the case of biology this is all you need to know.
    In the case of more advanced biology. I think many animals should in principle be evolutionary susceptible for polarised light because organisms are more ordered. However I have not yet hear of this. maybe insects?
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    edd
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    Bees do, and a number of sea creatures do (plus many more I won't be aware of - I'm a physicist not a biologist). The mantis shrimp can rather famously detect polarisation, but that's because its eyes are frankly mildly insane - with crazy numbers of colour pigments (giving it more than the standard number of primary colours in effect), circular as well as plane polarisation detection in some species, eyes on stalks that can move independently and trinocular (not just binocular) vision with just one eyestalk.

    They're also quite tasty.
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    Quote Originally Posted by edd View Post
    They're also quite tasty.
    Taste the electromagnetic superiority...
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