Why in this video does the 2nd black hole appears to change size and appear larger the farther away it gets? How can you see behind it? http://www.youtube.com/watch?v=ENd8Sz0AFOk
Answer
Black holes bend light passing them and this means they act as a lens. The phenomenon is called gravitational lensing. The way black holes bend light is different to the way a conventional lens, for example in a magnifying glass, bends light and as a result there can be some very odd visual effects. In this case the lensing by the black hole at the front is magnifying the black hole at the back and distorting it into an Einstein ring.
Working out exactly what the gravitational lensing does is exceedingly complicated. If anyone is interested some details of the calculation shown in this image are described in this paper.
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