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.