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I was laying on my bed, reading a book when the sun shone through the windows on my left. I happened to look at the wall on my right and noticed this very strange effect. The shadow of my elbow, when near the pages of the book, joined up with the shadow of the book even though I wasn't physically touching it.

I have posted a video of the event here. (Note: the video seems to be wrong way up but you still get the idea of what is happening)

What is causing this? Some sort of optical illusion where the light get's bent? Coincidentally, I have been wondering about a similar effect recently where if you focus your eye on a nearby object, say, your finger, objects behind it in the distance seem to get curved/distorted around the edge of your finger. It seems awfully related...

EDIT: I could see the bulge with my bare eyes to the same extent as in the video! The room was well light and the wall was indeed quite bright.

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Great question. You really put some effort into it :-) A well deserved +1. –  John Rennie Jan 18 at 19:35
@JohnRennie Agreed; a nice example of looking at Physics in everyday life and appreciating its beauty. Nice answer too, John! –  Ahaan Rungta Jan 18 at 23:38
my first response to seeing this question was to frantically attempt to replicate it. With little luck :D –  codedude Jan 19 at 5:47
As for the second part of your question, it is light getting bent, but it isn't an illusion. It's a phenomenon called diffraction, where an object of small dimensions causes any wave to bend around it. :) You can read up about it on the internet. :) –  mikhailcazi Jan 19 at 7:19
@mikhailcazi Ah, I didn't think diffraction would be happening on such a level. I guess I'm used to small apertures. I will look into the exact on-goings when I get a chance :) –  PPG Jan 19 at 8:57
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6 Answers

up vote 65 down vote accepted

As said by John Rennie, it has to do with the shadows' fuzzyness. However, that alone doesn't quite explain it.

Let's do this with actual fuzzyness:

Fuzzy overlapping shadows

I've simulated shadow by blurring each shape and multiplying the brightness values1. Here's the GIMP file, so you can see how exactly and move the shapes around yourself.

I don't think you'd say there's any bending going on, at least to me the book's edge still looks perfectly straight.

So what's happening in your experiment, then?

Nonlinear response is the answer. In particular in your video, the directly-sunlit wall is overexposed, i.e. regardless of the "exact brightness", the pixel-value is pure white. For dark shades, the camera's noise surpression clips the values to black. We can simulate this for the above picture:

Nonlinear response of overlapping shadows

Now that looks a lot like your video, doesn't it?

With bare eyes, you'll normally not notice this, because our eyes are kind of trained to compensate for the effect, which is why nothing looks bent in the unprocessed picture. This only fails at rather extreme light conditions: probably, most of your room is dark, with a rather narrow beam of light making for a very large luminocity range. Then, the eyes also behave too non-linear, and the brain cannot reconstruct how the shapes would have looked without the fuzzyness anymore.

Actually of course, the brightness topography is always the same, as seen by quantising the colour palette:

Brightness isobares

1To simulate shadows properly, you need to use convolution of the whole aperture, with the sun's shape as a kernel. As Ilmari Karonen remarks, this does make a relevant difference: the convolution of a product of two sharp shadows $A$ and $B$ with blurring kernel $K$ is

$$\begin{aligned} C(\mathbf{x}) =& \int_{\mathbb{R}^2}\!\mathrm{d}{\mathbf{x'}}\: \Bigl( A(\mathbf{x} - \mathbf{x}') \cdot B(\mathbf{x} - \mathbf{x'}) \Bigr) \cdot K(\mathbf{x}') \\ =& \mathrm{IFT}\left(\backslash{\mathbf{k}} \to \mathrm{FT}\Bigl(\backslash\mathbf{x}' \to A(\mathbf{x}') \cdot B(\mathbf{x}') \Bigr)(\mathbf{k}) \cdot \tilde{K}(\mathbf{k}) \right)(\mathbf{x}) \end{aligned} $$

whereas seperate blurring yields

$$\begin{aligned} D(\mathbf{x}) =& \left( \int_{\mathbb{R}^2}\!\mathrm{d}{\mathbf{x'}}\: A(\mathbf{x} - \mathbf{x}') \cdot K(\mathbf{x}') \right) \cdot \int_{\mathbb{R}^2}\!\mathrm{d}{\mathbf{x'}}\: B(\mathbf{x} - \mathbf{x'}) \cdot K(\mathbf{x}') \\ =& \mathrm{IFT}\left(\backslash{\mathbf{k}} \to \tilde{A}(\mathbf{k}) \cdot \tilde{K}(\mathbf{k}) \right)(\mathbf{x}) \cdot \mathrm{IFT}\left(\backslash{\mathbf{k}} \to \tilde{B}(\mathbf{k}) \cdot \tilde{K}(\mathbf{k}) \right)(\mathbf{x}). \end{aligned} $$

If we carry this out for a narrow slit of width $w$ between two shadows (almost a Dirac peak), the product's Fourier transform can be approximated by a constant proportional to $w$, while the $\mathrm{FT}$ of each shadow remains $\mathrm{sinc}$-shaped, so if we take the Taylor-series for the narrow overlap it shows the brightness will only decay as $\sqrt{w}$, i.e. stay brighter at close distances, which of course surpresses the bulging.

And indeed, if we properly blur both shadows together, even without any nonlinearity, we get much more of a "bridging-effect":

Blurred after combining the shadows

But that still looks nowhere as "bulgy" as what's seen in your video.

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The second picture would be a great graphic concept for a video game! –  Vortico Jan 19 at 3:37
I guess that phenomena doesn't occur for high-def. cameras. Am I right? Or, is it a property for all detectors? –  Waffle's Crazy Peanut Jan 19 at 4:25
This is a really good answer. I wish it was mine :-) –  John Rennie Jan 19 at 8:10
@Waffle'sCrazyPeanut: at some level of exposure, it will occur for any camera — even the best CCD chips have a limited dynamic range. But yes, if you set up a good camera correctly then it should behave sufficiently linear so you don't notice the effect. –  leftaroundabout Jan 19 at 13:02
I don't think this is the full answer. The thing is, convolution (including blurring) and multiplication don't commute, so you'll get different results if you merge the layers before you blur them (which is closer to the real situation, assuming the book and elbow are at approximately the same distance from the wall). And indeed, doing it "correctly" should generally make the "bridge" region between the shapes darker than in your example. –  Ilmari Karonen Jan 19 at 13:51
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It's because the Sun is not a point source, so the edges of the shadows are slightly fuzzy. This is my rather crude attempt to show why this happens:


There are far better diagrams in the Wikipedia article on the umbra that explains what is going on. The fuzzy bit at the edge of the shadow is called the penumbra.

The reason you see the bulge where the shadows approach is due to the penumbra and the fact that the human eye isn't that great at handling contrast. As the two shadows approach, but before they touch, their penumbras (penumbrae?) overlap. This means the region between the shadows is darker than the rest of the penumbra. Another rather crude diagram follows:


This isn't a great diagram because the density of the penumbra isn't constant, but rather shades from black to white across its width. However Google Draw doesn't do gradient fills so I'm stuck with a rather poor representation. Anyhow, it should hopefully be obvious that where the penumbras overlap they darken each other, so the region between the two shadows gets darker. Because the eye isn't good at handling a wide contrast range it looks as if the shadows have grown a bulge towards each other.

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The last sentence, wide contrast range, is crucial here, for normally our eyes are quite capable of doing the "deconvolution" needed to extrapolate the original shapes without any bulges. –  leftaroundabout Jan 19 at 0:20
(Actually, our brains, not our eyes, I suppose) –  leftaroundabout Jan 19 at 0:32
The sun is so far from Earth, that for any purpose it can be considered a point source. –  sammy Jan 19 at 19:05
@sammy That's generally a good rule of thumb for other stars (although it still can't be a blanket statement), but not for the Sun. It has a large enough angular diameter that it certainly can't always be approximated away to a point source. It depends on the situation. –  ajp15243 Jan 20 at 5:49
@sammy there is a diffraction of about half a degree on average, and I have seen the used mostly as a parallel source –  ratchet freak Jan 20 at 9:23
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I believe you'll find the cause is diffraction, as described in this article. The photo shows a similar effect when holding two fingers close together.

Black Drop Effect described in Sky and Telescope

As you bring your elbow and book together, you're creating a diffraction pattern as can be seen in the image in the article below, a brighter light in the middle with black bands on either side. As you bring them even closer together, the black bands grow closer together. This is why they appear to suddenly "jump" towards each other.

Image of standard diffraction pattern

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This is the only physically correct answer - I do not understand why completely unrelated answers get so many upvotes. –  sammy Jan 20 at 13:27
I can see the same effect with shadows on my living room wall lit by the long life bulb in the room. Since this not an even approximately coherent light source I'm disinclined to believe it's diffraction. However there is an easy test. If it's diffraction it will still be visible with a point source of light, but if it's overlap of penumbrae it will disappear if a point source is used. The two theories predict exactly opposite behaviour - if only all experimental physics was as clear cut! If I can find something to act as a point source I will attempt the experiment. –  John Rennie Jan 20 at 17:42
@JohnRennie This has me thinking a lot now. Some people say it is diffraction, some say it is penumbrae. Hmm. If it was diffraction wouldn't there be a diffraction pattern formed of light, not one formed from a shadow? The bulge seen in my video does look a lot like a diffraction pattern which may be confusing people. –  PPG Jan 20 at 19:27
An easy way to have a perfect point source is a cardboard with a very small hole in it. Put it in the window in a sunny day. –  sammy Jan 21 at 9:06
@JohnRennie diffraction affects all light, whether coherent or not. When it is coherent, you see nice fringes, and when it is not - just blurring, like in this example. –  gigacyan Jan 29 at 12:48
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Coincidentally, I have been wondering about a similar effect recently where if you focus your eye on a nearby object, say, your finger, objects behind it in the distance seem to get curved/distorted around the edge of your finger. It seems awfully related...

You are right they are related. This is another diffraction effect. You are seeing single edge diffraction, also known as the knife edge effect. Here is a web page describing it.

Light passing very near your finger is deflected. Since the direction has changed, it appears to come from a different place.

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I do not guarantee this is the correct answer, but you can verify it practically.

The light rays causing the shadow of your hand and that of your book, are not parallel. The bulging of shadows as they are near each other, means that both the bulged shadows are of the elbow. So, at the point when your elbow is near your book, two light sources (or bending of light such that unparallel rays of light act on the elbow causing two shadows) are acting on the elbow.

I think you too have guessed this.You can verify it by doing two things:

  1. Opening the window so that direct light can fall. This will tell if bending is caused by the glass window.
  2. Change of height of the book and the elbow wrt. the window. Try to see if the effect is observed at all heights.
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Try this out in a room that has one of those ceiling fans with four light bulbs. The multiple sources of light produce multiple shadows that are clearly visible and you can see this sort of phenomenon take effect. As the shadows overlap it gives this appearance and produces the phenomena spoken of. In normal situations the multiple sources of light encountered can often times be light that is reflected off of walls and nearby objects, anything that is white or lighter colored. The multiple shadows produced are so faint they are not even noticeable until they overlap other shadows. Sources of light from different angles produce multiple shadows. When your elbow moves toward the book, for example, multiple shadows from both objects overlap where they appear to combine. What looks like the edge of the shadow of a single object is the edge of where the layers of shadows overlap each other the most.

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