What is Gravitational Lensing? What is the effect known as gravitational lensing? How does it work? What objects would be able to cause this effect?
Gravitational lensing is the effect that large amounts of gravity have on the path of light. Here's an example of the effect:



The animation shows a black hole passing in front of a galaxy (simulated). In theory any object with a surface mass density larger than the critical surface mass density would be able to produce similar such effects.

What's happening is that light goes in a straight line along the spacetime fabric. When a very massive object is in the fabric, it bends the fabric - since the light follows a straight line on the fabric, the line is also bent. This means that the light goes in a not-straight line, and thus a lensing effect.