Geisha and the moving shadow


On a summer evening seating in my apartment in Baltimore, I noticed this character popping up on the sheer curtain moving by the wind. The shape morphed constantly and were read as different characters. From Geisha to a demon..a lady with a large bag pack and anything you could imagine of. I started photographing them as it appeared so interesting!

And I wondered how does this actually occur?

Objects cast shadows and move along with the moving objects in the scene, due to which object shape distortion occurs in the foreground, they typically differ significantly from the background.

Shadows are due to the occlusion of the light source by an object in the scene, which here is the figure of Geisha seating on the window frame. The part of an object that is not illuminated is called self-shadow (less lit areas of the figure), while the area projected on the curtain by the figure is called cast shadow and is further classified into umbra and penumbra. The umbra corresponds to the area where the direct light is totally blocked by the object, whereas in the penumbra area it is partially blocked.

Notice that the frontal silhouette of the Geisha in shadow remains constant except the change in overall scale while the rear silhouette gets stretched or pulled in.

Refer to the figure shown below to further understand umbra and penumbra.

Note that it is the angular size of the source that really matters. Here the source is the sun which is huge, but since it is so distant its angular diameter is only half a degree.

Hope this information was easy enough to grasp for you all!

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About anar memon

I am a person with a vast subject interests, call it culture, food, music, history, places, films, sports, design, art & crafts, painting, photography, animals, geography, architecture and the list never ends but keeps growing. A lot of times I try to intervine any two or more interests to get inspired and apply rules of one to the other and find interesting results. I'm always observing and learning from the world around me as I think one life is just not enough to grasp the beauty of our world.

4 comments

  1. Very interesting. In that image, the size of penumbra appears very small relative the size of umbra. I think that’s because we’re looking at their (partial) projection on a flat surface. It’s like looking at a cross-section of penumbra & umbra, and the cross-section happens to be real close to the object. But as we move away from the object, in the opposite direction from the source of light, the umbra will keep shrinking and eventually merge into a point (like a cone), while the penumbra will continue to expand and will stretch to infinity.

  2. Thanks for reading Vishal! I guess you are right. I can also look at it as stop motion graphic effect if at all I understood it right, where penumbra is the reaction or projection of constantly moving source of light..or constantly moving object. Correct me if I’m wrong.

  3. I don’t think motion is pertinent in understanding this phenomenon. (Motion does affect the size and shape of umbra/penumbra, but that’s about it.) Here’s how I conceive this: when the observer is in the umbra region, she can’t see the source of light; it’s completely hidden behind the object (like a full eclipse). But when the observer is in the penumbra region, only a portion of the source of light is hidden behind the object (like a partial eclipse).

    I think this picture from Wikipedia does a good job of throwing some, ahem, light on this shadowy concept:
    http://en.wikipedia.org/wiki/File:Geometry_of_a_Lunar_Eclipse.svg

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