What is the focal length of a strong gravitational lens?
by Budgieye moderator
If the nearest lens occurs near galaxies about 300,000 light-years away, is that the focal length of a lens?
by psaha scientist
Sort of ... gravitational lenses don't have a single focal length, but whatever distance an observer is at, can be a kind of focal length for them.
An ordinary convex lens will take two rays that are initially parallel, and deflect them so that they meet some distance away, and that distance is what we call the focal length. (A concave lens deflects rays outwards, as if they were coming from a single focal point.) Good lenses are designed so that the focal length is pretty much the same, no matter which part of the lens the light rays go through.
For gravitational lenses, the focal length depends on which part of lens the light is going through. Light going through the inner regions, with the strongest gravitational fields and the most warped space, will focus at a comparatively short distance. So in the nearest strong gravitational lenses (such as the Einstein cross) the rays that are coming together at our location have come through the inner part of the galaxy. For very distant lenses, it's the light that has gone through the outer regions of the galaxy that presents multiple images to us.