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  1. #1
    RalphLambrecht's Avatar
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    inverse ssquare law

    This is confusing me
    according to the inverse square lawB=I/d^2,theIllumination from a light sourcequadruples every time the distance from subject to light source is cut in half.Inconsequence doesn't that mean that the light source approaches infinite intensitywhen the distance to the light source approaches '0'?Hoew can this be?is there a flaw in the inverse square lawor is it limited to certain conditions?
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  2. #2
    Dr Croubie's Avatar
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    ooh, physics!
    yeah, you're somewhat right, basically it's asymptotic. Half the distance and you quadruple the intensity. Keep halving and you keep quadrupling, until you're infinitely close and light is infinitely intense.
    But as with most physics, there is a basic assumption, in that it's a point light source, which don't exist in reality. Get closer to the bulb and the bulb gets comparatively "larger".

    Even if you consider a single excited electron emitting single photons as it changes excitation levels, as you get closer and closer to within a few nm you enter the realm of quantum physics (where my 1st-year knowledge ends) and the whole thing falls apart. I think you'll need a Sheldon for a GUT or String Theory explanation past that...
    An awful lot of electrons were terribly inconvenienced in the making of this post.

    f/64 and be there.

  3. #3
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    I did a lot of sound wave stuff in my studies (Physics-- I didn't do as much with light or optics). Sound Pressure uses a very similar formula, and the baseline is measure at 1 meter.

    It's a valid formula, that does in fact approach infinity, as it cannot be divided by 0. The formula assumes a true point source, though, so the area of illumination at such fractional distances would be nill. If you were measuring light hitting a spec of dust, you could get it much closer to the point source before it wouldn't be illuminated. It assumes a total illumination of the subject, which is constant, through the distance.
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  4. #4
    fretlessdavis's Avatar
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    Quote Originally Posted by Dr Croubie View Post
    ooh, physics!
    yeah, you're somewhat right, basically it's asymptotic. Half the distance and you quadruple the intensity. Keep halving and you keep quadrupling, until you're infinitely close and light is infinitely intense.
    But as with most physics, there is a basic assumption, in that it's a point light source, which don't exist in reality. Get closer to the bulb and the bulb gets comparatively "larger".

    Even if you consider a single excited electron emitting single photons as it changes excitation levels, as you get closer and closer to within a few nm you enter the realm of quantum physics (where my 1st-year knowledge ends) and the whole thing falls apart. I think you'll need a Sheldon for a GUT or String Theory explanation past that...
    You beat me to it!

    But yes, the law applies with the same size subject, and assuming a point light source.

    Physics is an exact science in a frictionless vacuum, once you bring it into the real world, the small inaccuracies are small enough to not really affect things at the distances we use in photography.
    New-ish convert to film.
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  5. #5
    MattKing's Avatar
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    It is much more helpful if you use the inverse square law to predict how light intensity decreases as distance from the source increases.

    That way you don't run into problems with the fact that the law only applies completely to impossibly small, point light sources.
    Matt

    “Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

    Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2

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    loss

    I am at a loss on how to determine how this relates to my 110-film Teenage Mutant Ninja Turtles flash unit.

  7. #7
    fretlessdavis's Avatar
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    Quote Originally Posted by MattKing View Post
    It is much more helpful if you use the inverse square law to predict how light intensity decreases as distance from the source increases.

    That way you don't run into problems with the fact that the law only applies completely to impossibly small, point light sources.
    Same thing with everything I did in acoustics. SPL is standardized at being measured at 1m for speakers and such... makes up for the errors in not having sound coming from a point source...
    New-ish convert to film.
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  8. #8
    Dr Croubie's Avatar
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    Quote Originally Posted by fretlessdavis View Post
    Physics is an exact science in a frictionless vacuum, once you bring it into the real world, the small inaccuracies are small enough to not really affect things at the distances we use in photography.
    hehehe, frictionless vacuum
    An awful lot of electrons were terribly inconvenienced in the making of this post.

    f/64 and be there.

  9. #9
    fretlessdavis's Avatar
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    Quote Originally Posted by Dr Croubie View Post
    EVERYTHING works in a frictionless vacuum, except, you know, just about everything.

    EDIT: I almost posted the same comic with my post. Gotta love XKCD.
    New-ish convert to film.
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  10. #10

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    Look at it this way---a certain amount of light is distributed over a sphere, and the intensity is the amount of light divided by the surface area of that sphere. If the distance were zero, what would that surface area be? Zero. So you really *would* have a light of infinite intensity at that point, in the "assume a spherical cow" world that the equation models.

    -NT
    Nathan Tenny
    San Diego, CA, USA

    The lady of the house has to be a pretty swell sort of person to put up with the annoyance of a photographer.
    -The Little Technical Library, _Developing, Printing, And Enlarging_

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