Question: What is the photon sphere?

Miranda Jackson answered on 17 Jun 2015:

Hi Ajeet, It was nice chatting with you earlier. As you may know, gravity can bend spacetime, and by doing this, it can make light travel a curved path. It the gravitational field is strong enough, as it is near a black hole, gravity can force light to travel endlessly in a circle around the black hole. The circle has a particular radius, depending on the mass of the black hole. If you consider the light travelling in all possible directions at this one radius, it traces out a sphere in 3-dimensional space. This is known as the photon sphere.