Quote:
Originally Posted by CraigS
I would say the only reason we see the object in the first place, is because Earth coincidentally, lies along the 'focal line' of the lens (as I mentioned before). There is no 'focal point'. 'Focal point' in optics has a very specific meaning. It is where light rays converge. There is no convergence. We merely lie along the path of propagation of the 'lensed' light. There is no 'convergence' and thus, there is no 'divergence'.
This is actually the distinguishing feature of a gravitational lens, and differentiates it from an optical lens.
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Hi Craig,
Firstly, let me say that I agree with you that gravitational lensing has a different mode of operation to classical optical lens systems.
There are differences but there are also similarities. And some of the terminology from classical optics does imply a similar meaning in the context of gravity-based lensing even if the mode of operation is different.
I'll concede on the divergence issue as this depends on properties of anti-matter that I can't comment on. If anti-matter does repel normal matter then many new doors are opened.
However, as to the issues on convergence and focus, consider the attached diagram of a symmetrical gravitational lens.
Every massive body is capable of lensing. In fact, given the right circumstances, this lensing can take the form of a ring of multiple images of the far body. In your argument, you are looking specifically at an observer at point A that sees light that has travelled on a specific path (or geodesic) from the far galaxy shown. The observer at A has no sense of the convergence of this light, nor of any specific focal point.
Now consider an observer on the Earth. This observer sees at least two images of the far galaxy, image 1 and image 2. The reason he/she sees these is that the paths of light from the distance galaxy converge to a point on the Earth. This is in fact the focal point for the gravitational lens. Every symmetrical lens will have a specific focal point. The focal point will depend on several factors e.g. the distance of the far galaxy and the mass of the lensing body.
Now consider an observer at B. He/she sees neither image 1 nor 2 as the focal point for this model is further out i.e the Earth.
This is of course an ideal situation. Many lenses are not symmetric and there will perhaps be several focal points (somewhat like astigmatism in classical optics).
And, although a different mode of operation, we can also use the term magnification with regard to gravitational lensing.
Regards, Rob.