Use the slider to change the mass of the central black hole. The light source is also movable by clicking and dragging!

Since the black hole bends spacetime, the shortest path for light to follow (called a *geodesic*) is curved around the black hole. So, light emitted at an angle away from the black hole can end up curving back around to the other side. This phenomenon is called *gravitational lensing* and is the reason it is possible to see the back side of a black hole while looking directly at it.

The white circle around the black hole (which is 3/2 the Schwarzschild radius/event horizon) represents the *photon sphere*, which is where light orbits the black hole rather than being sucked in or escaping off. This means that if you stood directly in the photon sphere, you'd able to see the back of your own head as photons from the back of your head orbit around to your eye!

Of course, this radius is extremely precise and small errors in this simulation mean that light doesn't quite orbit, even if we snap the star to emit light at exactly the right radius.