A question for all you physics professors (or hobby physicists) - holiday season edition

Discussion in 'Open Discussion' started by quatchi, Dec 20, 2015.

  1. quatchi

    quatchi Mu-43 Veteran

    May 17, 2012
    Munich, Germany
    Here is a question related to physics (optics to be precise). And in order to fit into the season it comes in a nice holiday wrapping... :) 

    The story starts with a christmas tree (with LED lights) which I set up on our balcony. Being bored one evening I remembered my early photography days (around 2009) when I first got the Pana 20mm and made a custom star shaped aperture for it. So I dug in our tinker box and came up with a reindeer puncher. With that puncher I made a custom aperture in form of that reindeer (for the Oly 45mm f1.8):


    With that I photographed the lights of our christmas tree yesterday evening. The "trick" is to have the lights out of focus so the lights are blurred and take the shape of the aperture (usually round but with the custom aperture reindeer shaped).

    Here are two pictures - the first with focus set roughly between the camera and the tree and the second with close focus on the lens.

    PC190016. PC190015.

    Ok, so far for the "background story". Lets come to the question.
    When I pixel-peeped on the reindeers I came across very interesting effects. There are at least three effects I didn't expect (at least not without having spend any thought on it so far).
    • A brighter border around the reindeers
    • "Ripple" effects inside the reindeers towards the border
    • "Rays" around the reindeers (best seen on the first close-up - looks almost like fur)
    Here are close-ups (100% crops) of the two pictures.

    Bildschirmfoto 2015-12-20 um 09.31.50.
    Bildschirmfoto 2015-12-20 um 09.32.18.

    There you have it. :)  My guess for the ripples is that is has something to do with refraction and the "rays/fur" with the paper/cardboard used for the custom aperture. But any proper explanation or insides on this would be very nice.

    Nice holiday season by the way. :) 
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  2. pdk42

    pdk42 One of the "Eh?" team

    Jan 11, 2013
    Leamington Spa, UK
    Well, I'm just an amateur, but I'll take a pop:

    - Ripples - interference patterns caused by diffraction at the edges of the reindeer mask
    - "Fur" - I'm guessing it's due to small fibres on the edges of the cutout. I suppose there's some additional diffraction caused by that
    - Brighter halo - hmm, maybe something akin to the airy-disk banding you get from a point source, but in this case the edge?

    Merry Christmas!
    Last edited: Dec 20, 2015
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  3. MoonMind

    MoonMind Mu-43 Top Veteran Subscribing Member

    Oct 25, 2014
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  4. pdk42

    pdk42 One of the "Eh?" team

    Jan 11, 2013
    Leamington Spa, UK
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  5. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    The ripple effect is just diffraction (refraction is light passing from one material to another like air/glass, air/water, etc.).

    Diffraction is not a simple blur but is a ripple like effect:

    File:Laser Interference.JPG - Wikipedia, the free encyclopedia
    File:Single Slit Diffraction (english).svg - Wikipedia, the free encyclopedia
    File:Water diffraction.jpg - Wikimedia Commons

    When you consider the effect on a flat surface placed in front of the wave after the diffraction happened there is one bigger peak of intensity and a series of smaller ones. This assumes only perfectly flat and parallel waves coming in something you'll never see in real world.

    All discussions about the "diffraction limit" usually refer approximately to the diameter of the main peak (Rayleigh criterion) that again depends on the specific wave length ("color") of the light.

    I agree with others that "fur" is due to the paper irregular border, micro cuts/slits causing diffraction that happens around any obstacle, not only "hole borders":


    Brighter halo is partly an illusion due to the black borders. I cut and pasted the border in the middle of the image and it is still a little brighter. I'd say diffraction again but I cannot really visualize an explanation (waves not parallel to the cardboard?).
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  6. drd1135

    drd1135 Zen Snapshooter

    Mar 17, 2011
    Southwest Virginia
    Actually, those are some great examples of diffraction, especially in that darker brown reindeer in the middle. It's a really nice and interesting shot.
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  7. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    To be more precise is not diffraction alone but as @MoonMind@MoonMind said there is also interference at play.
    Diffraction is the waves "spreading around the borders after they hit an obstacles" while interference is the "peaks and troughs of different waves cancelling each other out (or adding up)".

    After a wave hit an obstacle it "changes direction" and starts to interfere with the diffracted waves from the other side and the central part of the wave that passed through the middle unaltered. Wth the water one wave pushes the water up and another one pushes the water down in the very same place. The result is a grid of places where the water remains at the base level and you have all the intermediate levels with doubled peaks and troughs.

    With light the "base level" places appear dark when projected on a screen.
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  8. eteless

    eteless Mu-43 All-Pro

    Jun 20, 2014
    The spikes/fur remind me a bit of diffraction spikes fwiw. Sort of related to all of the above... I'm guessing the edge of the punch has machining marks which is why there's a regular pattern.
  9. quatchi

    quatchi Mu-43 Veteran

    May 17, 2012
    Munich, Germany
    Thanks for your replies! Optics is a very interesting field. I guess I have some reading to do over the holidays... :) 
  10. Djarum

    Djarum Super Moderator Subscribing Member

    Dec 15, 2009
    Huntsville, AL, USA
    Instead of a round airy disk we end up with a reindeer airy disk...lol.
  11. WT21

    WT21 Mu-43 Legend Subscribing Member

    Feb 19, 2010
    Since your question is already answered, just to throw in my two cents -- thanks for posting! A great idea I never heard of, and might try with the kids :) 
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  12. tim63

    tim63 Mu-43 Regular

    Sep 5, 2015
    For goodness sake, it's Christmas magic!

    Diffraction? Pah!!!
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