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Homework Help: Light Phenomena - shadows within shadows

  1. Jun 1, 2007 #1
    I'm new to the forum, so please guide me along. (This is my first post)

    1. The problem statement, all variables and given/known data

    This isn't really part of a homework assignment, just something I observed, and am puzzled as to why it occurs:

    Shadows seem to stick to each other, as illustrated in the picture below
    http://www.inaneasylum.org/users/Photoshag/Default/sd.jpg [Broken]

    A shadow within a shadow produces an elongated shape

    What I observed was when there are two objects, object A, B
    1) If Object A is closer to the light source than object B
    2) And the ojects are positioned such that their shadows meet
    3) Then, the shadow of Object B appeaprs to stick to the shadow of Object A

    To better show what I mean, I uploaded a video of it on youtube:

    Here's another video:

    Sorry for the bad quality of the videos, hope it illustrates the phenomena adequetely.

    2. Relevant equations

    I don't know any equations, maybe there is one that relates the [shape of the elongated shadow] to the [distance between the objects and light source] and the [distance between the two cast shadows]. But this is too complex for me to find. (My mathematics isn't that good)

    3. The attempt at a solution

    I know that this merging of shadows has something to do with diffraction, but I am unsure as to how to properly explain it, to me it seems an interesting phenomena, with probably quite a simple explanation behind it. (Perhaps placing a camera between the objects and the light source will reveal what is going on)

    I don't think it's an optical illusion. The variation in tones of gray, and the gradient may contribute to it being an optical illusion, but I don not think that that is the only reason.

    Hope you guys can help me with this, it should work if you try it at home. :smile:
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Jun 1, 2007 #2
    Well, welcome to Physics Forums! The 'sticking' phenomena is due to diffraction.

    EDIT: Oh; :P you said it was diffraction. Well, the reason in simplistic terms is that the light spreads. Thus the concentration of light decreases over a given area. Thus a shadow appears, giving the 'sticking effect.' Otherwise I don't see what you are asking...
    Last edited by a moderator: Jun 1, 2007
  4. Jun 2, 2007 #3
    Hey, thanks for the reply, could you elaborate more about how the diffraction works, I still don't understand. Is diffraction what caused the shadow in the picture to be elongated?

    It seems weird that when a shadow enters another shadow it stretches in such a way.

    How about if the two shadows were each cast from a different light source, in theory would this effect still occur?

    I tried it using two torchlights, and it didn't seem to work.

    Thanks again for your help :smile:
  5. Jun 2, 2007 #4
    I think you can use theory to hypothesize and figure out exactly what happens. Remember that diffraction happens within a slit... Thus try this:

    try varying the distance of the circle and the square board. By distance, I mean as measured from the light source or the surface upon which the shadow falls upon. If the difference in distances is small, then the effect (I think) should be more visible. If the difference is large, then the effect becomes less and less visible.
  6. Jun 2, 2007 #5
    I just tried out your suggestion, and what I saw was quite different.

    When the distances were small, meaning that both objects were close to the floor (the surface upon which the shadows were cast), the shadows cast were sharp, and naturally the effect was less visible.

    When the distance between both objects was small, less "sticking" occured; when the objects were side by side, there was almost no merging of the shadows at all.

    It seemed that the effect became more visible when the difference in distances was large.

    Just to summarise, the effect is most visible when the objects are far away from each other, and far away from the surface upon which their shadows are cast.
  7. Jun 2, 2007 #6
    Interesting... Okay Can you please try one more.

    Try large difference in distance again.

    This time I reckon the shadow of the object that is further from the light source merges towards the shadow of the object closest to the light source.

    Anyway the result of this can be enough to come up with a sufficient theory...
  8. Jun 2, 2007 #7
    Yup, what you reckoned was right, you figured it out already?

    I think I know what's going on under normal circumstances, with strong light, (just to note I observed most of the stuff with shadows cast from sunlight), but what's captured in the picture had me stumped.

    The first picture on top was taken when a passing cloud was covering the sun, and thus caused the light intensity to decrease.

    [What I'm saying is that under strong lighting, it is near impossible to observe a shadow within a shadow, but the picture shows just this; so under weaker lighting does the whole shadow become a transparent gradient aloowing us to see shadows on top of shadows?]

    Does the effects of diffraction increase when the intensity of light decreases, seems weird, but I do not know how else to explain it.

    Sorry for the long post... I think I'm confusing myself.
  9. Jun 2, 2007 #8
    I think that the matter of "approach the slit to the screen" didn't work because the waves had less "space" to interact.


    In the second image of that link, we can imagine what happens if one approach the screen to the double slit, enought close there is not interference.

    In the second question, I think that the small shadow between the big shadows is "weaker" than the big shadows themselves. So when the "intensity" of your extense focus increase, you have more light everywhere, making the central shadow near invisible because the reflexions in the surface by other beams.

    Try to make that in a dark room with only a slit letting the light enter, and then prepare your experiment with this slit. Later vary the intensity of the initial beam... there should not be difference.
  10. Jun 2, 2007 #9
    Sorry, i'm not quite sure what you mean.

    Are you saying that the shadow within the other shadow is caused because of light bouncing off the surroundings?

    So do you think that the area at which the [elongated part of the circular shadow is] is at the place between the gradients of the two shadows or at the place where the two solid shadows themselves are.

    It seems that when the light intensity decresed, the gradient or the spreading of the shadow increased; so is this increase in spreading due to the light intensity or just light bouncing off other surfaces and the object's surfaces?

    I do not understand what you mean about the slit, with a true point source, there would be no diffraction right? No umbra or gradient, just sharp shadows, and thus no "sticking" of the shadows, with a slit an interference pattern of light and dark would be seen. (If it was monochrome light I think) So why use a slit?
  11. Jun 2, 2007 #10
    No, I'm saying that the "sticked shadow" is created by the small space between the two objects, but that this shadow is more "gray" than the other two, and when you have light coming from an extense focus, then if that light grows in intensity, the interference becomes less visible due the light bouncing that dosen't go trought the "slit" created by the two objects.

    My sugerence of doing the experiment in a dark room is to eliminate the unwanted light that spoils the experiment. This is, if you get a dark room and make a hole of the size that you really need (letting only some light enter) you can do a better study.

    Try with different sizes of the hole, I expect that, if it is enought big to let pass light to interfere in your "slit" (space between the objects), the results will be the same but easier to see. Also keep the objects enought far from the screen, so the light can interfere.
    Last edited: Jun 2, 2007
  12. Jun 2, 2007 #11
    THIS IS NOT MY BEST DRAWING:smile: - I AM FAR BETTER - however, in 1 minute, this is the best I can produce.

    The black lines are light waves.

    The Red lines are the objects.

    The blue lines show the merging effect...

    EDIT: Okay this image is really bad. Just try to get the idea...

    Attached Files:

  13. Jun 2, 2007 #12
    Oh ok, I get what you mean, but I think the diffraction here works in quite a different way.

    The "sticked shadow" is not caused by a slit, because the object closer to the light source has to cast a shadow on the 2nd object (further from the light source) before any sticking would occur.

    That is to say that if the objects were seen parallelly (i'm not sure if there's such a word), there would be no gap between the objects for light to pass through.

    If you saw the second video, it would seem that there was a gap, but there actually isn't, the camera and even the human eye is probably not sensitive enough to see the entire large gradient surrounding both shadows.

    EDIT: I haven't seen the image you attached, so I do not know whether what I said above still holds, will reply when it has been approved and I can view it.
    Last edited: Jun 2, 2007
  14. Jun 2, 2007 #13
    I think a conclusion has been reached.
  15. Jun 2, 2007 #14
    Oh, NOW I know what do you say, but still I'm not sure if the increase of intensity (and decrease of the shadow) will happen if the experiment is made in more optimal conditions. Maybe in this case is due to some sort of incident angle of the light. But I've not ideas.
  16. Jun 2, 2007 #15
    I observed some stranger things, and you guys have evry reason to doubt this, but I'll post it anyway:

    I illustrated what I saw with photoshop:

    http://www.inaneasylum.org/users/Photoshag/Default/shadows.jpg [Broken]

    If you look closely at the first picture above, you'll see that the shadow isn't really "elongated" as I had mentioned (It isn't a shaped produced from the stretching of the first shadow); rather, the second shadow seems to poke out of the first, and is much closer to that illustrated in the last panel above.

    The weird thing is that I have only observed it a few times, and it seemed to work out in such a strange way that I thought my eyes were playing tricks, how could a second shadow extend and connect in such a way. I asked my sister and dad to take a look, and they saw what I did.

    I hope you guys would try it out for yourselves, not under strong lighting conditions, because this post by itself is unreliable uinless you see what I saw.

    You know what I thought at first? :rolleyes: I was thinking that shadows can attract each other, and the shadow cast from the object further away from the light source had a weaker force of attraction, thus was attracted to the second shadow, which had a stronger force. This is so much gibberish, and it's illogical, but I really thought it was that way.
    Last edited by a moderator: May 2, 2017
  17. Jun 3, 2007 #16
    Maybe. Just make sure that this experiment is reproducable. Then you could probably (easily) it with modifications to my diagram.
  18. Jun 3, 2007 #17
    hmm... is there any other way I can view your diagram?
    Right now it still says "Attachment Pending Approval"... Maybe email it to me?
    unicoated@yahoo.com if you have the time

    Thanks lots :smile:
  19. Jun 3, 2007 #18
    Thanks for sending me the image, now I know what you're saying, but I'm a bit confused when you said "because of the way waves tend to spread out in the direction they travel in, when an orpaque obstacle arises, they spread in a different direction"

    Waves spread in a different direction when they meet an obstacle? Is this part of the process of diffraction? I know that they would bounce off the obstacle, is this what you mean by change direction? cos its quite different from what's illustrated in your diagram. I did a web search and couldn't find much info on waves changing direction.
    Last edited: Jun 3, 2007
  20. Jun 3, 2007 #19
    I tried to say that the main concentration changes into a different direction. It is the concept of reflection; the way it can be explained with diffraction. I'll send another attachment...
  21. Jun 3, 2007 #20
    Well this is my attachment...:rolleyes:

    Attached Files:

  22. Jun 4, 2007 #21
    do attachments ever get approved here? your previous image attachment is still pending approval...

    Anyways, I think I understand how it works, I read up on diffraction and other properties of light, and what's happening in the image does have a simple explanation... all linked to diffraction

    Thanks to all you guys who helped me on this problem :smile:
  23. Jul 7, 2007 #22
    Solution : )

    lol, been quite long since this thread has had a new post in it.
    I juz received an email asking if I could explain the phenomena, made me realise that I just said I figured it out, but never really posted the solution/ conclusion. so here it is (with help from prasannapakkiam's attachments)

    The phenomena as mentioned is caused by diffraction, a common feature of waves.

    It occurs when wavelengths interfere with each other, and more importantly waves can bend or diffract around objects.

    Here is a picture explaining what is happening in the 1st pic on this forum.
    http://www.inaneasylum.org/users/Photoshag/Default/sd.jpg [Broken]

    image of diffraction:
    http://www.inaneasylum.org/users/Photoshag/Default/lightbend1.jpg [Broken]
    Just an exagerated rough sketch, the waves don't actually bend so rapidly.

    http://www.inaneasylum.org/users/Photoshag/Default/lightbend2.jpg [Broken]
    The gap between the purple lines and the red lines is where diffraction occurs.
    Light from the sun bends around the square object causing the elongated shadow of the circular object.

    How does it cause the elongated shadow? The square object is further from the floor, and as the previous previous image shows, the further an object is from the surface, the more diffraction would occur. This means the edges would be lighter and have a large gradient as more light has a chance to bend around the square object and hit the surface.

    The circular object is closer to the floor, thus less diffraction occurs (as seen from the darker edges). Thus light rays that have bent around the square object do not bend as much around the circular object this causes the dark area which gives the illusion of sticky shadows.

    Hope the explanation is clear. I haven't really learnt about diffraction in school, so pls correct me if any part of my post is wrong.

    Thx lots : )
    Last edited by a moderator: May 3, 2017
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