# 4: Light Orbits


• 4.1: Impact Parameter
In this chapter we will investigate the paths that photons take in Schwarzschild spacetime. That may seem like a silly question at first since it is often said that light travels in straight lines, but remember that General Relativity messes with the rules of geometry, so photons can travel in what appears to us as a curved path.
• 4.2: Effective Potential
In principle, we can use it along with the angular equation from the previous section to completely determine the motion of a photon in Schwarzschild spacetime. In practice, however, this is basically impossible to do by hand, which means we must either use a computer or use the qualitative effective potential energy technique that we discussed earlier.
• 4.3: Lensing
When a photon passes a massive object, the path of the photon is deflected. The photon may be deflected and collide with the massive object (or fall into the singularity), or the photon may "swing around" the massive object (possibly multiple times) and escape. In the last section, we discussed a method to get qualitative information about the path. In this section, we will look at how to get quantitative information about the light path under very specific conditions.
• 4.4: Video Resources

Thumbnail: Einstein cross: four images of the same astronomical object, produced by a gravitational lens. Image used wtih permission (Public Domain; NASA and ESA).​​​​​​