Gravitational Fields
Since gravitational fields incorporate acceleration, Einstein postulates that the same concepts must also apply to these gravitational fields. This is the strong equivalence principle. We must also remember that light always travels in straight lines through space-time, thus, it is gravity which curves space-time. The curvature of space-time is the main principle in Einstein’s theory of general relativity.
Gravitational Lensing is a phenomenon where an object is placed between a light source and an observer. Normally, the light would never reach the observer due to light traveling in straight lines, but due to the gravity of the object, the light is bent, allowing the observer to see an image of the light source which is not in the observers direct line of sight. The following diagram helps to explain this.
Here is an actual photo of gravitational lensing.
