Bending of light by a gravitational force
The Principle of Equivalence leads immediately to the recognition that a gravitational force can bend the path of a beam of light.
In each of the two systems above a beam of light is dispatched horizontally across the laboratory.
Thinking about system B (from the perspectives of an inertial observer, standing outside) it is clear that, as the lift accelerates upwards, relative to B’s laboratory the beam will bend downwards, below the horizontal.
The principle of equivalence then tells us that this must also be true in system A!
If you follow this thread still further you come to gravitational lensing