# Why do shadows seem to attract one another? **URL:** \ **Category:** Factual Questions\ **Created:** [May 23, 2002, 8:25pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540 "2002-05-23T20:25:08Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![michel](https://avatars.discourse-cdn.com/v4/letter/m/ea666f/32.png) [@michel](https://boards.straightdope.com/u/michel)\ **Post date:** [May 23, 2002, 8:25pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/1 "2002-05-23T20:25:08Z")
For example, when you stand against the sun and watch your shadow as you bring it near another… yours seems to pull away at the nearest point of contact to touch the other shadow. What causes this? Does it have to do with the electromagnetic properties of light? Or is it just my imagination 😛 Thanks! ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [May 23, 2002, 8:39pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/2 "2002-05-23T20:39:00Z")
Welcome to the SDMB! What you are describing is just your imagination. Light does bend around objects but the effect can only be seen around massive objects such as the sun. Your mass is totally insufficient to produce a measurable bending of light. ---
**Author:** ![Lance\_Turbo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lance_turbo/32/6156_2.png) [@Lance\_Turbo](https://boards.straightdope.com/u/Lance_Turbo)\ **Post date:** [May 23, 2002, 8:42pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/3 "2002-05-23T20:42:47Z")
Railroad tracks don’t attract each other either, but they appear to get closer together when you stand between them and look down their length. ---
**Author:** ![Neptune](https://avatars.discourse-cdn.com/v4/letter/n/b5a626/32.png) [@Neptune](https://boards.straightdope.com/u/Neptune)\ **Post date:** [May 23, 2002, 8:50pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/4 "2002-05-23T20:50:39Z")
Light does bend around small objects. It’s called difraction. When two objects are close difraction can lead to an interference pattern. I think this is the source of the phenomonon mentioned in the original post. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [May 23, 2002, 9:02pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/5 "2002-05-23T21:02:52Z")
> [@](#): > > \*Originally posted by Neptune \* > \*\*Light does bend around small objects. > It’s called difraction. > > When two objects are close difraction can lead to an interference pattern. > I think this is the source of the phenomonon mentioned in the original post. \*\* Correct. Light bends around all edges. So the edge of a shadow isn’t a clean, sharp line but rather is fuzzy because light rays that just barely graze the edge bend around it a little bit. When two objects are close to each other a slit is formed and there is an interference pattern caused by the difference in path length of the diffraction around the two objects causing the shadows, and the two shadows merge. Diffraction also causes a uni-directional radio antenna to have a small, residual sensitivity to waves coming from directly behind it. ---
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [May 23, 2002, 9:03pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/6 "2002-05-23T21:03:05Z")
I suspect that you have seen a genuine phenomenon–provided you are not claiming that the shadows leap together from inches (or even centimeters) apart. Most objects that are illuminated from a sufficiently wide light source will have a sort of “double” shadow called a penumbra. It is not too hard to understand. Picture (draw if you need to) the rays from the right hand side of a large light source being blocked by an object. A line drawn from the light to one side of the object and another line drawn to the other side of the object would form an angle. A separate line drawn from the left side of the light source to each side of the object would form a different angle. The place where those two angles overlap is the shadow. The place where the one on the right side provides shade but the left side does not provide shade (and vice versa) is the penumbra. When two objects (each with their own shadows and penumbrae) are moved near each other, at some point the penumbra from each object overlaps the penumbra from the other object and the combined penumbrae are nearly as dark as the normal shadow, making it appear that the two shadows are “reaching” for each other or are flowing together. Any light source wider than a pin-prick will produce this effect to _some_ extent. The sun, being rather wide (though apparently small because of distance) can provide this sort of optical effect rather well. Whether my explanation or **Neptune** ’s provides _more_ of the merging effect I will leave to people who actually dabble in the physics of light and optics. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [May 23, 2002, 9:04pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/7 "2002-05-23T21:04:37Z")
I’ve noticed this; shadows seem to ‘stick’ to one another - I think Neptune is correct. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [May 23, 2002, 9:05pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/8 "2002-05-23T21:05:50Z")
It’s not imagination, but it is misinterpretation (it’s certainly not perspective!) Every shadow has two parts, the _umbra_ and the _penumbra_. The umbra is the deep inner shadow, wherein all light is blocked. The penumbra is a lighter shadow, wherein part of the light is blocked (think of a partial solar eclipse). If you hold a baseball in front of a flashlight, the penumbra is huge; a small object in front of the very-far-away sun has a tiny penumbra that you barely notice. But if you overlap the penumbra of one object with the penumbra of another, a full umbra is formed, that looks like the shadows “reaching out” to each other. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [May 23, 2002, 9:32pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/9 "2002-05-23T21:32:11Z")
> [@](#): > > \*Originally posted by David Simmons \* > **Light bends around all edges.** I don’t think so. Light travels in a straight line. Period. Only massive\* objects that bend space itself (ala the sun) cause light to ‘bend’ and even that bending is somewhat of a misnomer. It is bending from our 3-D perspective but is following a straight line by its own reckoning. I think **tomndebb** and **nametag** have sussed it with the penumbra explanation. [sub]\*Technically any mass bends space but only really massive objects bend space anywhere near noticeably enough for our ability to measure it.[/sub] ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [May 23, 2002, 9:40pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/10 "2002-05-23T21:40:44Z")
I also meant to mention that I’m sticking with imagination as well being the issue here. At no time does the shadow actually move of its own accord to touch another shadow. What seems to eb happening is merely an optical illusion or a misperception (i.e. your imagination…not reality). Some of you have even contradicted yourselves (I’ll pick on **Nametag** here but I think you’ll see it in other posts as well). Bolding is mine. > [@](#): > > \*Originally posted by Nametag \* > It’s not imagination… > > _\_ > > … **looks like** the shadows **“reaching out”** to each other. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [May 23, 2002, 9:54pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/11 "2002-05-23T21:54:55Z")
Actually, on reflection (no pun intended), I’ll withdraw my support for the diffraction theory above; the penumbra effect seems the more plausible. You can achieve a similar effect (similar, but not quite the same) if you look at (and focus your eyes on) a distant source of light, and bring your fingertips together about an inch away from your eye - the unfocused ‘edge’ of each finger will appear to ‘leap’ across to join the other (but it’s the addition of the two unfocused areas that you’re really seeing). **Whack-a-Mole** ; what about diffraction though? (as a phenomenon, not as a cause of the effect that the OP mentions). ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [May 23, 2002, 9:58pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/12 "2002-05-23T21:58:07Z")
> [@](#): > > \*Originally posted by Whack-a-Mole \* > \*\* > > Light travels in a straight line. Period. \*\* This is the SDMB so we only deal in Straight Dope when it comes to the facts. A direct quote from the physics text, _Physics_, Hausman and Slack, D. Van Nostrand Co., Inc., New York: “Although it is commonly said that light travels in straight lines, careful observations show that it bends slightly around the edges of an obstruction. The spreading of a beam of light into the region behind and obstacle is called _diffraction._” On thinking about this some more, and going outside to look at some actual shadows I think there might be three effects, at least, that cause real shadows to have fuzzy edges. 1) The umbra-penumbra thing; 2) diffraction; and 3) the fact that some of the light that illuminates the edge has been scattered by the atmosphere so it hasn’t all travelled from the sun to the edge of the object by a straight, geometric line. In any case, I think we can all look at shadows from the sun and agree that the edges are fuzzy and the overlap of these fuzzy edges from two objects that are close to each other causes the shadows to merge before the objects actually touch. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [May 24, 2002, 1:56pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/13 "2002-05-24T13:56:06Z")
Obviously physics texts have a lot to say about this and I am nowhere near qualified enough to second guess them (and even those who are qualified wouldn’t second guess this as it seems pretty well established). That said I still don’t get it. I don’t see how a photon clipping along says, “Oh look, there’s an edge just beneath me. I think I’ll change course 5[sup]0[/sup] to the right!” What is it about an edge that causes a photon to change direction when it won’t do so any other time? What mechanism is pulling the photon off its original course? It sounds like free energy to me. Further, if I can bend light in this fashion why don’t I see people making boxes to cause light to turn corners? It would seem that all I’d need to do is create a box with a lot of partitions staggered slightly back from the one before it such that a photon meets and edge, turns a bit, meets another edge and turns some more and so on. You could make a new fangled sky light to bring sunlight from a low hanging sun into a room in this fashion. Again, I’m not arguing against what science has clearly learned so far but obviously I’m missing something and I wonder what it is. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [May 24, 2002, 2:26pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/14 "2002-05-24T14:26:35Z")
> [@](#): > > \*Originally posted by Whack-a-Mole \* > \*\* > That said I still don’t get it. I don’t see how a photon clipping along says, “Oh look, there’s an edge just beneath me. I think I’ll change course 5[sup]0[/sup] to the right!” \*\* In this case use waves, not photons. Early on in the study of light there were two views of “what light is,” the corpuscular (like photons) and the wave view. The phenomenon of diffraction was widely considered as one of the things that drove a stake through the heart of the corpuscular theory. Nobody could figure out how a discrete corpuscle of light could do that, while a wave explanation was available. Now, both views are considered valid and the one is used that is most useful for the purpose at hand. Isaac Asimov’s book _Understanding Physics_ has a pretty good explanation based on using Huygens’ Principle. This principle states that a wave front can be considered as a series of points on a line. Each of those points is postulated to be a source of a spherical wave. If you take another point slightly ahead (in the direction of propogation), the wave amplitude there is the sum of the spherical wave from all the points on the line. In free space that summation results in uniform waves that travel radially out from a point source. However, the edge introduces a discontinuity so that in making the summation, light energy from points on one side of the edge contribute to the summation but there is no light energy coming from points on the other side of the edge because they are blocked by the object. The result is that the wave front bends around the edge. This isn’t a “why” explanation but does show how you can make an analytical prediction of the effect. There really isn’t any “why” explanation extant that I know of. Sort of like “why” do objects exert a gravitational force on each other? They just do. Light just plain acts that way. ---
**Author:** ![breaknrun](https://avatars.discourse-cdn.com/v4/letter/b/bbce88/32.png) [@breaknrun](https://boards.straightdope.com/u/breaknrun)\ **Post date:** [May 24, 2002, 7:47pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/15 "2002-05-24T19:47:35Z")
I’m not a physics geek but it seems to me that light _has_ to bend around objects. If it didn’t, I don’t think there would be a penumbra at all and all shadows would be sharply defined. ---
**Author:** ![Neptune](https://avatars.discourse-cdn.com/v4/letter/n/b5a626/32.png) [@Neptune](https://boards.straightdope.com/u/Neptune)\ **Post date:** [May 24, 2002, 7:54pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/16 "2002-05-24T19:54:17Z")
The penumbra is a result of the width of a light source. For example, the sun is not a point source. It has a visible width, you can see it (but never look directly at the sun!). Imagine that you’re standing outside on a sunny day and you’re looking down at your shadow. Now imagine that you’re an ant on the ground in the shadow. An ant in the middle of the shadow cannot see the sun at all. An ant outside the shadow can see all the sun. But an any sitting on the edge of the shadow may be able to see just a part of the sun. The shadow isn’t so dark at that point. ---
**Author:** ![Omphaloskeptic](https://avatars.discourse-cdn.com/v4/letter/o/bcef8e/32.png) [@Omphaloskeptic](https://boards.straightdope.com/u/Omphaloskeptic)\ **Post date:** [May 24, 2002, 11:19pm UTC](https://boards.straightdope.com/t/why-do-shadows-seem-to-attract-one-another/110540/17 "2002-05-24T23:19:43Z")
> [@](#): > > \*Originally posted by Whack-a-Mole \* > \*\*Obviously physics texts have a lot to say about this and I am nowhere near qualified enough to second guess them (and even those who are qualified wouldn’t second guess this as it seems pretty well established). > > That said I still don’t get it. I don’t see how a photon clipping along says, “Oh look, there’s an edge just beneath me. I think I’ll change course 5[sup]0[/sup] to the right!” What is it about an edge that causes a photon to change direction when it won’t do so any other time? What mechanism is pulling the photon off its original course? It sounds like free energy to me. Further, if I can bend light in this fashion why don’t I see people making boxes to cause light to turn corners? It would seem that all I’d need to do is create a box with a lot of partitions staggered slightly back from the one before it such that a photon meets and edge, turns a bit, meets another edge and turns some more and so on. You could make a new fangled sky light to bring sunlight from a low hanging sun into a room in this fashion.\*\* **David Simmons** has already given an explanation of why diffraction happens. As to why it’s not used… well, it is, sort of. Diffraction only has a noticeable effect on the propagation of light close to the edge: within a few wavelengths. This explains why the effect isn’t widely used to direct visible light (wavelengths ~5x10[sup]-7[/sup]m). But diffraction is closely related to interference (“interference” is usually used when there are discrete sources as in the two-slit experiment, and “diffraction” when the source is better viewed as a continuous region), and multiple-source interferometers are very useful. Radiotelescope arrays and phased-array antennas use interference to direct their beam, for example. “Diffraction gratings” also use this effect.